[ELF] Place SectionPiece::{Live,Hash} bit fields together

Summary:
We access Live and OutputOff (which may share the same memory location)
concurrently in 2 parallelForEachN loops. Separating them avoids subtle
data races like D41884/PR35788. This patch places Live and Hash
together.

2 reasons this is appealing:

1) Hash is immutable. Live is almost read-only - only written once in MarkLive.cpp where
   Hash is not accessed
2) we already discard low bits of Hash to decide ShardID. It doesn't
   matter much if we make 32-bit Hash to 31-bit.

   For a huge internal clang -O3 executable (1.6GiB),
   `Strings` in StringTableBuilder::finalizeStringTable contains at most 310253 elements.
   The expected number of pair-wise collisions 2^(-31) * C(310253,2) ~= 22.41 is too small to have a negative impact on performance.
   Actually, my benchmark shows there is actually a minor performance improvement.

Differential Revision: https://reviews.llvm.org/D60765

llvm-svn: 358645