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:

git-svn-id: 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed