add a comment explaining why darwin/i386 uses ## as a comment.

It's not clear why this is really required, but it was explicitly
added in r48808 with no real explanation or rdar #.

llvm-svn: 96438

llvm/lib/Target/X86/X86MCAsmInfo.cpp

@@ -55,6 +55,8 @@ X86MCAsmInfoDarwin::X86MCAsmInfoDarwin(const Triple &Triple) {
   if (!is64Bit)
     Data64bitsDirective = 0;       // we can't emit a 64-bit unit
 
+  // Use ## as a comment string so that .s files generated by llvm can go
+  // through the GCC preprocessor without causing an error.
   CommentString = "##";
   PCSymbol = ".";