[clang][NFC] Wrap TYPE_SWITCH in "do while (0)" in the interpreter

Wraps the expansions of TYPE_SWITCH and COMPOSITE_TYPE_SWITCH in
the constexpr interpreter with "do { ... } while (0)" so that these
macros can be used like this:

if (llvm::Optional<PrimType> T = Ctx.classify(FieldTy))
  TYPE_SWITCH(*T, Ok &= ReturnValue<T>(FP.deref<T>(), Value));
else
  Ok &= Composite(FieldTy, FP, Value);

This bug was found while testing D116316. See also review comment:
https://reviews.llvm.org/D64146?id=208520#inline-584131

Also cleaned up the macro definitions by removing the superfluous
do-while statements and removed the unused INT_TPYE_SWITCH macro.

Differential Revision: https://reviews.llvm.org/D117301