[clang][bytecode] Fix memcmp/bcmp failures on big-endian hosts (#119851)

See the discussion in

#119678 (comment)
and
#119544 (comment)

Author: tbaederr

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -1943,10 +1943,18 @@ static bool interp__builtin_memcmp(InterpState &S, CodePtr OpPC,
   BitcastBuffer BufferA(
       Bits(S.getASTContext().getTypeSize(PtrA.getFieldDesc()->getType())));
   readPointerToBuffer(S.getContext(), PtrA, BufferA, false);
+  // FIXME: The swapping here is UNDOING something we do when reading the
+  // data into the buffer.
+  if (S.getASTContext().getTargetInfo().isBigEndian())
+    swapBytes(BufferA.Data.get(), BufferA.byteSize().getQuantity());
 
   BitcastBuffer BufferB(
       Bits(S.getASTContext().getTypeSize(PtrB.getFieldDesc()->getType())));
   readPointerToBuffer(S.getContext(), PtrB, BufferB, false);
+  // FIXME: The swapping here is UNDOING something we do when reading the
+  // data into the buffer.
+  if (S.getASTContext().getTargetInfo().isBigEndian())
+    swapBytes(BufferB.Data.get(), BufferB.byteSize().getQuantity());
 
   size_t MinBufferSize = std::min(BufferA.byteSize().getQuantity(),
                                   BufferB.byteSize().getQuantity());

clang/lib/AST/ByteCode/InterpBuiltinBitCast.cpp

@@ -73,11 +73,6 @@ using DataFunc =
     }                                                                          \
   } while (0)
 
-static void swapBytes(std::byte *M, size_t N) {
-  for (size_t I = 0; I != (N / 2); ++I)
-    std::swap(M[I], M[N - 1 - I]);
-}
-
 /// We use this to recursively iterate over all fields and elements of a pointer
 /// and extract relevant data for a bitcast.
 static bool enumerateData(const Pointer &P, const Context &Ctx, Bits Offset,

clang/lib/AST/ByteCode/InterpBuiltinBitCast.h

@@ -19,6 +19,11 @@ class InterpState;
 class CodePtr;
 class Context;
 
+inline static void swapBytes(std::byte *M, size_t N) {
+  for (size_t I = 0; I != (N / 2); ++I)
+    std::swap(M[I], M[N - 1 - I]);
+}
+
 bool DoBitCast(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
                std::byte *Buff, Bits BitWidth, Bits FullBitWidth,
                bool &HasIndeterminateBits);