[find-all-symbols] Parallelize the merge step.

There is still more parallelism to get here because we synchonize on the
actual uniquing but just doing YAML parsing in parallel already gives a
significant speedup.

Merging all symbols in LLVM+clang+compiler-rt+lld+libc++, 48 cores.
before: 201.55s user 1.47s system 99% cpu 3:23.04 total
after:  276.99s user 7.63s system 838% cpu 33.947 total

Differential Revision: http://reviews.llvm.org/D19720

llvm-svn: 268037