llvm-dwp: Add an abstraction for the DWP string pool

Also reference strings in the memory mapped file, reduces memory usage
on a large test case by 18.5%.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270449 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed