[clang] [clang] Implement CWG2428 "Deprecating a concept" (PR #92295)

[Aaron Ballman via cfe-commits]

================
@@ -45,6 +45,34 @@ void fallthrough(int n) {
 #endif
 }
 
+namespace cwg2428 { // cwg2428: 19
+#if __cplusplus >= 202002L
+template <typename>
+concept C [[deprecated]] = true; // #C
----------------
AaronBallman wrote:

I'd like an additional test for rejecting:
```
template <typename>
[[deprecated]] concept C = true;
```
which isn't allowed per the grammar (though I find this slightly surprising and it may be an oversight from WG21).

It would also be good to have a test explicitly showing that we reject non-C++-style attributes.

https://github.com/llvm/llvm-project/pull/92295