[Symbolizer] Implement pc element in symbolizing filter.

Implements the pc element for the symbolizing filter, including it's
"ra" and "pc" modes. Return addresses ("ra") are adjusted by
decrementing one. By default, {{{pc}}} elements are assumed to point to
precise code ("pc") locations. Backtrace elements will adopt the
opposite convention.

Along the way, some minor refactors of value printing and colorization.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D131115