Explicitly ban async in trait impls

This wouldn't compile before because the return type wouldn't match,
but now it's properly an error.

Author: cramertj

src/librustc_passes/ast_validation.rs

@@ -265,6 +265,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     self.invalid_visibility(&impl_item.vis, None);
                     if let ImplItemKind::Method(ref sig, _) = impl_item.node {
                         self.check_trait_fn_not_const(sig.header.constness);
+                        self.check_trait_fn_not_async(impl_item.span, sig.header.asyncness);
                     }
                 }
             }

src/test/ui/did_you_mean/issue-40006.stderr

@@ -38,11 +38,11 @@ LL |       L = M; //~ ERROR missing
 LL | |     Z = { 2 + 3 }; //~ ERROR expected one of
    | |____^ missing `fn`, `type`, or `const`
 
-error: expected one of `const`, `extern`, `fn`, `type`, `unsafe`, or `}`, found `;`
+error: expected one of `async`, `const`, `extern`, `fn`, `type`, `unsafe`, or `}`, found `;`
   --> $DIR/issue-40006.rs:23:18
    |
 LL |     Z = { 2 + 3 }; //~ ERROR expected one of
-   |                  ^ expected one of `const`, `extern`, `fn`, `type`, `unsafe`, or `}` here
+   |                  ^ expected one of 7 possible tokens here
 
 error: expected one of `!` or `::`, found `(`
   --> $DIR/issue-40006.rs:24:9

src/test/ui/token/issue-41155.stderr

@@ -1,8 +1,8 @@
-error: expected one of `(`, `const`, `default`, `extern`, `fn`, `type`, or `unsafe`, found `}`
+error: expected one of `(`, `async`, `const`, `default`, `extern`, `fn`, `type`, or `unsafe`, found `}`
   --> $DIR/issue-41155.rs:13:1
    |
 LL |     pub
-   |        - expected one of 7 possible tokens here
+   |        - expected one of 8 possible tokens here
 LL | } //~ ERROR expected one of
    | ^ unexpected token