make ConstString allocate memory in non-tiny chunks

BumpPtrAllocator allocates in 4KiB chunks, which with any larger
project is going to result in a large number of allocations.
Increasing allocation size this way can save 10%-20% of symbol
load time for a huge C++ project with correctly built debuginfo.

Differential Revision: https://reviews.llvm.org/D68549

llvm-svn: 374583