[clang] [Parser][NFC] Move the core parsing of an attribute into a separate method (PR #107300)

[Aaron Ballman via cfe-commits]

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@@ -2943,6 +2943,9 @@ class Parser : public CodeCompletionHandler {
     return false;
   }
 
+  bool ParseGNUSingleAttribute(ParsedAttributes &Attrs, SourceLocation &EndLoc,
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AaronBallman wrote:

This could be read as parsing `__attribute__((foo))` as opposed to just `foo` alone. However, I'm not certain I have a better name. `ParseSingleGNUAttributeAfterIntroducer` was the best I could come up with.

If that doesn't make anyone happy, can we switch to `ParseSingleGNUAttribute()` instead?

https://github.com/llvm/llvm-project/pull/107300