Optionally print symbolizer markup backtraces.

When the environment LLVM_ENABLE_SYMBOLIZER_MARKUP is set, if
llvm-symbolizer fails or is disabled, this change will print a backtrace
in llvm-symbolizer markup instead of falling back to in-process
symbolization mechanisms.

This allows llvm-symbolizer to be run on the output later to produce a
high quality backtrace, even for fully-stripped LLVM utilities.

Reviewed By: mcgrathr

Differential Revision: https://reviews.llvm.org/D139750