Fix llvm-for-windows-on-linux build after LLVM r272701.

The file is called "intrin.h". When building targeting Windows on a Linux
system, with the SDK mounted in a case-insensitive file system, "Intrin.h" will
miss clang's intrin.h header (because that's not in a case-insensitive file
system) but then find intrin.h in the Microsoft SDK. clang can't handle the
SDK's intrin.h.

https://reviews.llvm.org/D36281

llvm-svn: 309980