[clang] Reland #90786 ([BoundsSafety] Allow 'counted_by' attribute on pointers in structs in C) (PR #93121)

[Dan Liew via cfe-commits]

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@@ -4993,20 +4993,8 @@ void Parser::ParseLexedCAttribute(LateParsedAttribute &LA,
          "late field attribute expects to have at most one declaration.");
 
   // Dispatch based on the attribute and parse it
-  const AttributeCommonInfo::Form ParsedForm = ParsedAttr::Form::GNU();
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delcypher wrote:

@hnrklssn @rapidsna Heads up I plan to land this change too. I discovered that this this switch statement doesn't need to exist because `ParseGNUAttributeArgs` effectively already does what we need.

https://github.com/llvm/llvm-project/pull/93121