Support -masm= flag for x86 assembly targets.

For assembly files without .intel_syntax or .att_syntax directives, allow the
-masm= flag to supply a default assembly dialect. For example,

C:\TMP> type intel.s
.text
mov al,0

C:\TMP> clang -masm=intel -c intel.s

Without this patch, one would need to pass an "-mllvm -x86-asm-syntax=" flag
directly to the backend.
C:\TMP> clang -mllvm --x86-asm-syntax=intel -c intel.s

Differentials Review: http://reviews.llvm.org/D22285

llvm-svn: 275877