[ARM] Set a non-conflicting comment character for assembly in MSVC mode

Currently, for ARMCOFFMCAsmInfoMicrosoft, no comment character is set, thus the
idefault, '#', is used.

The hash character doesn't work as comment character in ARM assembly, since '#'
is used for immediate values.

The comment character is set to ';', which is the comment character used by MS
armasm.exe. (The microsoft armasm.exe uses a different directive syntax than
what LLVM currently supports though, similar to ARM's armasm.)

This allows inline assembly with immediate constants to be built (and brings the
assembly output from clang -S closer to being possible to assemble).

A test is added that verifies that ';' is correctly interpreted as comments in
this mode, and verifies that assembling code that includes literal constants
with a '#' works.

Patch by Martin Storsjö.

llvm-svn: 276859