Fix thread race between SectionPiece's OutputOff and Live members

[dim]

Summary

As reported in bug 35788, rL316280 reintroduces a race between two
members of SectionPiece, which share the same 64 bit memory location.

Reduce the Hash member by 1 bit, and move the Live member next to it,
instead, while making OuputOff use the full 64 bit again.

Adjust getShardId to cope with Hashes being 31 bits wide (I hope I
interpreted that function correctly... :)

[ruiu]

LGTM

[smeenai]

Should this be considered for picking to the 6.0 release branch?

lld/trunk/ELF/SyntheticSections.cpp

void MergeNoTailSection::finalizeContents() {
   if (ThreadsEnabled)
     Concurrency =
         std::min<size_t>(PowerOf2Floor(hardware_concurrency()), NumShards);
 
   // Add section pieces to the builders.
   parallelForEachN(0, Concurrency, [&](size_t ThreadId) {
     for (MergeInputSection *Sec : Sections) {
       for (size_t I = 0, E = Sec->Pieces.size(); I != E; ++I) {
-        if (!Sec->Pieces[I].Live)
-          continue;
         size_t ShardId = getShardId(Sec->Pieces[I].Hash);
-        if ((ShardId & (Concurrency - 1)) == ThreadId)
+        if ((ShardId & (Concurrency - 1)) == ThreadId && Sec->Pieces[I].Live)
           Sec->Pieces[I].OutputOff = Shards[ShardId].add(Sec->getData(I));
       }
     }
   });
 
   // Compute an in-section offset for each shard.
   size_t Off = 0;
   for (size_t I = 0; I < NumShards; ++I) {