This adds a parser for the log symbolizer markup format discussed in
https://discourse.llvm.org/t/rfc-log-symbolizer/61282. The parser
operates in a line-by-line fashion with minimal memory requirements.
This doesn't yet include support for multi-line tags or specific parsing
for ANSI X3.64 SGR control sequences, but it can be extended to do so.
The latter can also be relatively easily handled by examining the
resulting text elements.
Reviewed By: peter.smith
Differential Revision: https://reviews.llvm.org/D124686
Daniel Thornburgh