clang-cl: Add support for asm listings (/FA and /Fa)

This adds support for outputing the assembly to a file during compilation.
It does this by changing the compilation pipeling to not use the integrated
assembler, and keep the intermediate assembler file.

Differential Revision: http://llvm-reviews.chandlerc.com/D1946

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@192902 91177308-0d34-0410-b5e6-96231b3b80d8