Fix a -Wformat warning in ASTUnit.cpp

llvm::sys::cas_flag is 'long' instead of 'uint32_t' on win32, because
that's what InterlockedIncrement is defined to accept.

I still don't know if we should be calling fprintf from ASTUnit.cpp
behind a getenv check.
Fix a -Wmicrosoft warning about an unrepresentable enum value

In MSVC, enums are always signed unless you explicitly specify the type.