Fix a span in parse_ty_bare_fn.

It currently goes one token too far.

Example: line 259 of `tests/ui/abi/compatibility.rs`:
```
test_abi_compatible!(fn_fn, fn(), fn(i32) -> i32);
```
This commit changes the span for the second element from `fn(),` to
`fn()`, i.e. removes the extraneous comma.

This doesn't affect any tests. I found it while debugging some other
code. Not a big deal but an easy fix so I figure it worth doing.

Author: nnethercote

compiler/rustc_ast/src/ast.rs

@@ -2126,7 +2126,8 @@ pub struct BareFnTy {
     pub ext: Extern,
     pub generic_params: ThinVec<GenericParam>,
     pub decl: P<FnDecl>,
-    /// Span of the `fn(...) -> ...` part.
+    /// Span of the `[unsafe] [extern] fn(...) -> ...` part, i.e. everything
+    /// after the generic params (if there are any, e.g. `for<'a>`).
     pub decl_span: Span,
 }
 

compiler/rustc_parse/src/parser/ty.rs

@@ -608,7 +608,7 @@ impl<'a> Parser<'a> {
             self.dcx().emit_err(FnPointerCannotBeAsync { span: whole_span, qualifier: span });
         }
         // FIXME(gen_blocks): emit a similar error for `gen fn()`
-        let decl_span = span_start.to(self.token.span);
+        let decl_span = span_start.to(self.prev_token.span);
         Ok(TyKind::BareFn(P(BareFnTy { ext, safety, generic_params: params, decl, decl_span })))
     }