Revert "[clang][Interp] Implement String- and CharacterLiterals"

This reverts commit 6fa1795d.

This breaks a windows builder:

../../clang/lib/AST/Interp/ByteCodeExprGen.h(236,11): error: call to constructor of 'clang::interp::APInt' (aka 'llvm::APInt') is ambiguous
    APInt WrappedValue(getIntWidth(Ty), Value, std::is_signed<T>::value);
          ^            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../clang/lib/AST/Interp/ByteCodeExprGen.cpp(409,16): note: in instantiation of function template specialization 'clang::interp::ByteCodeExprGen<clang::interp::ByteCodeEmitter>::emitConst<unsigned int>' requested here
  return this->emitConst(E, E->getValue());
               ^
../../clang/lib/AST/Interp/ByteCodeExprGen.cpp(1149,16): note: in instantiation of member function 'clang::interp::ByteCodeExprGen<clang::interp::ByteCodeEmitter>::VisitCharacterLiteral' requested here
template class ByteCodeExprGen<ByteCodeEmitter>;
               ^
../../llvm/include\llvm/ADT/APInt.h(108,3): note: candidate constructor
  APInt(unsigned numBits, uint64_t val, bool isSigned = false)
  ^
../../llvm/include\llvm/ADT/APInt.h(134,3): note: candidate constructor
  APInt(unsigned numBits, unsigned numWords, const uint64_t bigVal[]);
  ^