[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275921 91177308-0d34-0410-b5e6-96231b3b80d8

lib/ProfileData/InstrProfWriter.cpp

@@ -182,6 +182,14 @@ Error InstrProfWriter::addRecord(InstrProfRecord &&I, uint64_t Weight) {
   return Dest.takeError();
 }
 
+Error InstrProfWriter::mergeRecordsFromWriter(InstrProfWriter &&IPW) {
+  for (auto &I : IPW.FunctionData)
+    for (auto &Func : I.getValue())
+      if (Error E = addRecord(std::move(Func.second), 1))
+        return E;
+  return Error::success();
+}
+
 bool InstrProfWriter::shouldEncodeData(const ProfilingData &PD) {
   if (!Sparse)
     return true;