[Windows] Symbolize with llvm-symbolizer instead of dbghelp in a self-host

Summary:
llvm-symbolizer understands both PDBs and DWARF, so it is more likely to
succeed at symbolization. If llvm-symbolizer is unavailable, we will
fall back to dbghelp. This also makes our crash traces more similar
between Windows and Linux.

Reviewers: Bigcheese, zturner, chapuni

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12884

llvm-svn: 252118