[clang] [clang] support Wold-style-declaration (PR #78837)

[Aaron Ballman via cfe-commits]

================
@@ -3348,6 +3348,7 @@ void Parser::ParseDeclarationSpecifiers(
   while (true) {
     bool isInvalid = false;
     bool isStorageClass = false;
+    bool isFunctionSpecifier = false;
----------------
AaronBallman wrote:

Hmmm, what's obsoleted are storage class specifiers that are not at the start of the declaration. e.g.,
```
int static i = 12;
```
and function specifiers are not storage class specifiers. So technically, this is not obsolete:
```
void _Noreturn func(void);
```
GCC does warn about this, and I think that's reasonable to also mimic. I'll check with WG14 whether function specifiers should also be obsoleted similar to storage class specifiers.

https://github.com/llvm/llvm-project/pull/78837