Refactor StringMap.h, splitting StringMapEntry out to its own header.

Summary:
StringMapEntry.h can have lower dependencies, than StringMap.h, which
is useful for public headers that want to expose inline methods on
StringMapEntry<> but don't need to expose all of StringMap.h.  One
example of this is mlir's Identifier.h, another example is the existing
LLVM StringPool.h.

StringPool also could use a cleanup, I'll deal with that in a follow-on
patch.

Reviewers: rriddle

Subscribers: hiraditya, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77963