Implement span quoting for proc-macros

This PR implements span quoting, allowing proc-macros to produce spans
pointing *into their own crate*. This is used by the unstable
`proc_macro::quote!` macro, allowing us to get error messages like this:

```
error[E0412]: cannot find type `MissingType` in this scope
  --> $DIR/auxiliary/span-from-proc-macro.rs:37:20
   |
LL | pub fn error_from_attribute(_args: TokenStream, _input: TokenStream) -> TokenStream {
   | ----------------------------------------------------------------------------------- in this expansion of procedural macro `#[error_from_attribute]`
...
LL |             field: MissingType
   |                    ^^^^^^^^^^^ not found in this scope
   |
  ::: $DIR/span-from-proc-macro.rs:8:1
   |
LL | #[error_from_attribute]
   | ----------------------- in this macro invocation
```

Here, `MissingType` occurs inside the implementation of the proc-macro
`#[error_from_attribute]`. Previosuly, this would always result in a
span pointing at `#[error_from_attribute]`

This will make many proc-macro-related error message much more useful -
when a proc-macro generates code containing an error, users will get an
error message pointing directly at that code (within the macro
definition), instead of always getting a span pointing at the macro
invocation site.

This is implemented as follows:
* When a proc-macro crate is being *compiled*, it causes the `quote!`
  macro to get run. This saves all of the sapns in the input to `quote!`
  into the metadata of *the proc-macro-crate* (which we are currently
  compiling). The `quote!` macro then expands to a call to
  `proc_macro::Span::recover_proc_macro_span(id)`, where `id` is an
opaque identifier for the span in the crate metadata.
* When the same proc-macro crate is *run* (e.g. it is loaded from disk
  and invoked by some consumer crate), the call to
`proc_macro::Span::recover_proc_macro_span` causes us to load the span
from the proc-macro crate's metadata. The proc-macro then produces a
`TokenStream` containing a `Span` pointing into the proc-macro crate
itself.

The recursive nature of 'quote!' can be difficult to understand at
first. The file `src/test/ui/proc-macro/quote-debug.stdout` shows
the output of the `quote!` macro, which should make this eaier to
understand.

This PR also supports custom quoting spans in custom quote macros (e.g.
the `quote` crate). All span quoting goes through the
`proc_macro::quote_span` method, which can be called by a custom quote
macro to perform span quoting. An example of this usage is provided in
`src/test/ui/proc-macro/auxiliary/custom-quote.rs`

Custom quoting currently has a few limitations:

In order to quote a span, we need to generate a call to
`proc_macro::Span::recover_proc_macro_span`. However, proc-macros
support renaming the `proc_macro` crate, so we can't simply hardcode
this path. Previously, the `quote_span` method used the path
`crate::Span` - however, this only works when it is called by the
builtin `quote!` macro in the same crate. To support being called from
arbitrary crates, we need access to the name of the `proc_macro` crate
to generate a path. This PR adds an additional argument to `quote_span`
to specify the name of the `proc_macro` crate. Howver, this feels kind
of hacky, and we may want to change this before stabilizing anything
quote-related.

Additionally, using `quote_span` currently requires enabling the
`proc_macro_internals` feature. The builtin `quote!` macro
has an `#[allow_internal_unstable]` attribute, but this won't work for
custom quote implementations. This will likely require some additional
tricks to apply `allow_internal_unstable` to the span of
`proc_macro::Span::recover_proc_macro_span`.

compiler/rustc_span/src/hygiene.rs

@@ -144,7 +144,10 @@ impl ExpnId {
             let expn_data = self.expn_data();
             // Stop going up the backtrace once include! is encountered
             if expn_data.is_root()
-                || expn_data.kind == ExpnKind::Macro(MacroKind::Bang, sym::include)
+                || matches!(
+                    expn_data.kind,
+                    ExpnKind::Macro { kind: MacroKind::Bang, name: sym::include, proc_macro: _ }
+                )
             {
                 break;
             }
@@ -839,7 +842,13 @@ pub enum ExpnKind {
     /// No expansion, aka root expansion. Only `ExpnId::root()` has this kind.
     Root,
     /// Expansion produced by a macro.
-    Macro(MacroKind, Symbol),
+    Macro {
+        kind: MacroKind,
+        name: Symbol,
+        /// If `true`, this macro is a procedural macro. This
+        /// flag is only used for diagnostic purposes
+        proc_macro: bool,
+    },
     /// Transform done by the compiler on the AST.
     AstPass(AstPass),
     /// Desugaring done by the compiler during HIR lowering.
@@ -852,7 +861,7 @@ impl ExpnKind {
     pub fn descr(&self) -> String {
         match *self {
             ExpnKind::Root => kw::PathRoot.to_string(),
-            ExpnKind::Macro(macro_kind, name) => match macro_kind {
+            ExpnKind::Macro { kind, name, proc_macro: _ } => match kind {
                 MacroKind::Bang => format!("{}!", name),
                 MacroKind::Attr => format!("#[{}]", name),
                 MacroKind::Derive => format!("#[derive({})]", name),