[llvm] Add assembly color highlighting

Add support for syntax highlighting assembly. The patch introduces new
RAII helper called WithMarkup that takes care of both emitting colors
and markup annotations. It makes adding markup easier and ensures colors
and annotations remain consistent.

This patch adopts the new helper in the AArch64 backend. If your backend
already uses markup annotations, adoption is as easy as using the new
MCInstPrinter::markup overload.

Differential revision: https://reviews.llvm.org/D159162