[clang] [clang][Interp] Implement __builtin_classify_type (PR #71972)

[Aaron Ballman via cfe-commits]

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@@ -439,6 +439,194 @@ static bool interp__builtin_popcount(InterpState &S, CodePtr OpPC,
   return true;
 }
 
+// Values returned by __builtin_classify_type, chosen to match the values
+/// produced by GCC's builtin.
+enum class GCCTypeClass {
+  None = -1,
+  Void = 0,
+  Integer = 1,
+  // GCC reserves 2 for character types, but instead classifies them as
+  // integers.
+  Enum = 3,
+  Bool = 4,
+  Pointer = 5,
+  // GCC reserves 6 for references, but appears to never use it (because
+  // expressions never have reference type, presumably).
+  PointerToDataMember = 7,
+  RealFloat = 8,
+  Complex = 9,
+  // GCC reserves 10 for functions, but does not use it since GCC version 6 due
+  // to decay to pointer. (Prior to version 6 it was only used in C++ mode).
+  // GCC claims to reserve 11 for pointers to member functions, but *actually*
+  // uses 12 for that purpose, same as for a class or struct. Maybe it
+  // internally implements a pointer to member as a struct?  Who knows.
+  PointerToMemberFunction = 12, // Not a bug, see above.
+  ClassOrStruct = 12,
+  Union = 13,
+  // GCC reserves 14 for arrays, but does not use it since GCC version 6 due to
+  // decay to pointer. (Prior to version 6 it was only used in C++ mode).
+  // GCC reserves 15 for strings, but actually uses 5 (pointer) for string
+  // literals.
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AaronBallman wrote:

Needs to be updated after https://github.com/llvm/llvm-project/pull/72036 lands; sharing this with ExprConstant.cpp would be even better though. It might make sense to have a local header in `clang/lib/AST/` for contents that are shared between `Interp/*` and `ExprConstant.cpp`, WDYT?

https://github.com/llvm/llvm-project/pull/71972