Reapply "[ADT][StringMap] Add ability to precompute and reuse the string hash"

Reverted due to an internally discovered lld crash, which turned out to
be an existing lld bug that got tickled by this changes. That's
addressed in dee8786f so let's have
another go with this change.

Original commit message:
    Useful for lldb's const string pool, using the hash to determine which
    string map to lock and query/insert.

    Derived from https://reviews.llvm.org/D122974 by Luboš Luňák

This reverts commit f976719f.
Effectively reapplying 67c631d2.