Make .asm a valid extension for assembly files

This is a common extension on Windows, and now clang will assemble them
instead of treating them as linker input which is the default for unknown
file types.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192919 91177308-0d34-0410-b5e6-96231b3b80d8