[clang] [Attributes] Support Attributes being declared as only supporting late parsing when passing an experimental feature flag (PR #88596)

[Dan Liew via cfe-commits]

================
@@ -91,11 +91,24 @@ static StringRef normalizeAttrName(StringRef Name) {
 
 /// isAttributeLateParsed - Return true if the attribute has arguments that
 /// require late parsing.
-static bool isAttributeLateParsed(const IdentifierInfo &II) {
+bool Parser::isAttributeLateParsed(const IdentifierInfo &II) {
+  // Some attributes might only be late parsed when in the experimental
----------------
delcypher wrote:

@erichkeane I'm a little confused. Isn't it used here:

```
      // Handle "parameterized" attributes
      if (!LateAttrs || !isAttributeLateParsed(*AttrName)) {
        ParseGNUAttributeArgs(AttrName, AttrNameLoc, Attrs, &EndLoc, nullptr,
                              SourceLocation(), ParsedAttr::Form::GNU(), D);
        continue;
      }
```

in 

```
void Parser::ParseGNUAttributes(ParsedAttributes &Attrs,
                                LateParsedAttrList *LateAttrs, Declarator *D) {
```

?

https://github.com/llvm/llvm-project/pull/88596