Introduce an initial sketch of a MSVC compatible 'intrin.h' builtin
header, along with a stub test to make sure it compiles in the
appropriate modes.

Thanks to Aaron Ballman for working with me to figure out the initial
strategy here, and to Nico for reviewing and pestering me to actually
commit it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@155425 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed