[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

lib/Target/ARM/MCTargetDesc/ARMMCAsmInfo.cpp

@@ -90,6 +90,7 @@ ARMCOFFMCAsmInfoMicrosoft::ARMCOFFMCAsmInfoMicrosoft() {
 
   PrivateGlobalPrefix = "$M";
   PrivateLabelPrefix = "$M";
+  CommentString = ";";
 }
 
 void ARMCOFFMCAsmInfoGNU::anchor() { }

test/MC/ARM/Windows/literals-comments.s

@@ -0,0 +1,13 @@
+; RUN: llvm-mc -triple armv7-windows-msvc -filetype obj -o - %s
+
+  .syntax unified
+  .thumb
+
+  .text
+
+  .global function
+  .thumb_func
+function:
+  ; this is a comment
+  mov r0, #42 ; this # was not
+  bx lr