Change ConstString to support massive multi-threaded access

Previously ConstString had a single mutex guarding the global string
pool for each access what become a bottleneck when using it with a
large number of threads.

This CL distributes the strings to 256 individual string pools based on
a simple hash function to eliminate the bottleneck and speed up the
multi-thread access.

The goal of the change is to prepare to multi-threaded symbol parsing code
to speed up the symbol parsing speed.

Differential revision: http://reviews.llvm.org/D13652

llvm-svn: 250289