Add option to use temporary file for assembling with clang

This commit adds the flag '-via-file-asm' to the clang driver. The
purpose of this flag is to have a way to test that clang can consume
the assembly code that it outputs. When passed this flag, clang will
generate a temporary file that contains the assembly output from the
compile step. This assembly file will then be consumed by either the
integrated assembler or the external assembler. To test that the
integrated assembler can consume its own output compile with:

  $ clang -integrated-assembler -via-file-asm

Without the '-via-file-asm' flag, clang would directly create the
object file when using the integrated assembler. With the flag it
will first create the temporary assembly file and then read that
file and assemble it with the integrated assembler.

The flow is similar to -save-temps, except that it only effects
the assembly input and the temporary file is not saved.

llvm-svn: 196606