[llvm-profdata] Speed up merging by using a thread pool
Add a "-j" option to llvm-profdata to control the number of threads
used. Auto-detect NumThreads when it isn't specified, and avoid spawning
threads when they wouldn't be beneficial.
I tested this patch using a raw profile produced by clang (147MB). Here is the
time taken to merge 4 copies together on my laptop:
No thread pool: 112.87s user 5.92s system 97% cpu 2:01.08 total
With 2 threads: 134.99s user 26.54s system 164% cpu 1:33.31 total
Differential Revision: https://reviews.llvm.org/D22438
llvm-svn: 275921