Fix a -Wformat warning in ASTUnit.cpp

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200718 91177308-0d34-0410-b5e6-96231b3b80d8