[dsymutil] Make NonRelocatableStringPool a wrapper around DwarfStringPoolEntry. NFC

This is needed in order to use our StringPool entries in the Apple
accelerator tables.

As this is NFC we rely on the existing tests for correctness.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323339 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed