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!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215615 91177308-0d34-0410-b5e6-96231b3b80d8