NFC: Clean up the implementation of StringPool a bit, and remove dependence on...

NFC: Clean up the implementation of StringPool a bit, and remove dependence on some "implicitly MallocAllocator" based methods on StringMapEntry. This allows reducing the #includes in StringMapEntry.h.

Summary:
StringPool has many caveats and isn't used in the monorepo.  I will
propose removing it as a patch separate from this refactoring patch.

Reviewers: rriddle

Subscribers: hiraditya, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77976