A few recent changes are causing build breaks when `-DLLVM_ENABLE_MODULES=ON` (such as 834dfd23155351c9885eddf7b9664f7697326946 and 7dfdca1961aadc75ca397818bfb9bd32f1879248). This PR makes the required updates so that clang/llvm builds when `-DLLVM_ENABLE_MODULES=ON`. rdar://140803058