This reverts commit 3e559509b426b6aae735a7f57dbdaed1041d2622 and e0c4ffa796b553fa78c638a9584c05ac21fe07d5.
This still breaks Windows builds.
In addition, `#include <llvm/ADT/StringViewExtras.h>` in
llvm/include/llvm/Demangle/ItaniumDemangle.h is a library layering violation
(LLVMDemangle is the lowest LLVM library and cannot depend on LLVMSupport).