[clang] [C23] Implement WG14 N3037 (PR #132939)

[Aaron Ballman via cfe-commits]

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@@ -11399,6 +11400,22 @@ static QualType mergeEnumWithInteger(ASTContext &Context, const EnumType *ET,
   return {};
 }
 
+QualType ASTContext::mergeTagTypes(QualType LHS, QualType RHS) {
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AaronBallman wrote:

I'm following the naming convention already used by other helpers: `mergeExceptionSpecs`. `mergeFunctionTypes`, `mergeFunctionParameterTypes`, and so on. The intent is that this will eventually do actual merging logic (so the name is reasonable), but even for right now, you either get the "merged" type (which is either `LHS` or `RHS`) or you get `QualType{}`. WDYT?

https://github.com/llvm/llvm-project/pull/132939