Include macro name in 'local ambiguity' error

Currently, we only point at the span of the macro argument. When the
macro call is itself generated by another macro, this can make it
difficult or impossible to determine which macro is responsible for
producing the error.

compiler/rustc_expand/src/lib.rs

@@ -1,6 +1,7 @@
 #![feature(crate_visibility_modifier)]
 #![feature(decl_macro)]
 #![feature(destructuring_assignment)]
+#![feature(format_args_capture)]
 #![feature(iter_zip)]
 #![feature(proc_macro_diagnostic)]
 #![feature(proc_macro_internals)]