Add support for tombstone symbolization to pbtombstone.

This patch teaches pbtombstone to use llvm-symbolizer to symbolize
stack traces and augment the protobuf tombstones with the symbol
information, before printing tombstones with the symbolized stack
traces included.

The main advantage of adding this information to the tombstone
as opposed to having developers use the stack tool is that stack
does not print all of the information in the original tombstone,
which means that both reports may be required to understand a crash.
Furthermore, stack traces printed by stack are not correlated with
the stack traces in the tombstone, making the report harder to read,
especially with GWP-ASan and MTE which may produce multiple stack
traces for the crashing thread.

Although we could teach stack to print more information, this would
continue to be fragile because stack relies on parsing textual
tombstones. Switching stack to read proto tombstones would be
tantamount to a full rewrite and would require duplicating the C++
proto-to-text logic that we already have in Python. It seems better
to reuse the C++ code for the proto-based symbolization tool.

llvm-symbolizer will look up the symbol files by build ID using a
.build-id directory following the standard here:
https://fedoraproject.org/wiki/RolandMcGrath/BuildID

It will look for .build-id directories under paths specified with
--debug-file-directory, which pbtombstone will pass through to
llvm-symbolizer using its own --debug-file-directory flag. The
intent is that tools for platform developers will pass the flag
--debug-file-directory $ANDROID_PRODUCT_OUT/symbols to pbtombstone.
Soong will start creating .build-id under symbols after a corresponding
Soong CL lands.

Bug: 328531087
Change-Id: Ia4676821cf980c69487cf11aefa2a02dc0c1626f