Introduce proc_macro_back_compat lint, and emit for time-macros-impl

Now that future-incompat-report support has landed in nightly Cargo, we
can start to make progress towards removing the various proc-macro
back-compat hacks that have accumulated in the compiler.

This PR introduces a new lint `proc_macro_back_compat`, which results in
a future-incompat-report entry being generated. All proc-macro
back-compat warnings will be grouped under this lint. Note that this
lint will never actually become a hard error - instead, we will remove
the special cases for various macros, which will cause older versions of
those crates to emit some other error.

I've added code to fire this lint for the `time-macros-impl` case. This
is the easiest case out of all of our current back-compat hacks - the
crate was renamed to `time-macros`, so seeing a filename with
`time-macros-impl` guarantees that an older version of the parent `time`
crate is in use.

When Cargo's future-incompat-report feature gets stabilized, affected
users will start to see future-incompat warnings when they build their
crates.

compiler/rustc_lint_defs/src/lib.rs

@@ -266,6 +266,7 @@ pub enum BuiltinLintDiagnostics {
     PatternsInFnsWithoutBody(Span, Ident),
     LegacyDeriveHelpers(Span),
     ExternDepSpec(String, ExternDepSpec),
+    ProcMacroBackCompat(String),
 }
 
 /// Lints that are buffered up early on in the `Session` before the