Initial naive implementation using Symbols to represent autodiff modes (Forward, Reverse)

Since the mode is no longer part of `meta_item`, we must insert it manually (otherwise macro expansion with `#[rustc_autodiff]` won't work).

This can be revised later if a more structured representation becomes necessary (using enums, annotated structs, etc).

Some tests are currently failing. I'll address them next.

compiler/rustc_span/src/symbol.rs

@@ -253,6 +253,7 @@ symbols! {
         FnMut,
         FnOnce,
         Formatter,
+        Forward,
         From,
         FromIterator,
         FromResidual,
@@ -348,6 +349,7 @@ symbols! {
         Result,
         ResumeTy,
         Return,
+        Reverse,
         Right,
         Rust,
         RustaceansAreAwesome,