[ms inline asm] Add support for label names with '$' chars

In MS inline asm syntax a label with '$' char produces an error, while in AT&T it does not.
In AT&T inline asm syntax Clang escapes the '$' char and replaces it with "$$". Adopted same approach for MS syntax.

Differential Revision: http://reviews.llvm.org/D15795

llvm-svn: 256545