MC: AsmLexer: handle multi-character CommentStrings correctly

As X86MCAsmInfoDarwin uses '##' as CommentString although a single '#' starts a
comment a workaround for this special case is added.

Fixes divisions in constant expressions for the AArch64 assembler and other
targets which use '//' as CommentString.

Patch by Janne Grunau!

llvm-svn: 215615