[Symbolize] Add log markup --filter to llvm-symbolizer.

This adds a --filter option to llvm-symbolizer. This takes log-bearing
symbolizer markup from stdin and writes a human-readable version to
stdout.

For now, this only implements the "symbol" markup tag; all others are
passed through unaltered. This is a proof-of-concept bit of
functionalty; implement the various tags is more-or-less just a matter
of hooking up various parts of the Symbolize library to the architecture
established here.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D126980