Reapply "lldb: Cache string hash during ConstString pool queries/insertions"

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

Original commit message:
    lldb was rehashing the string 3 times (once to determine which StringMap
    to use, once to query the StringMap, once to insert) on insertion (twice
    on successful lookup).

    This patch allows the lldb to benefit from hash improvements in LLVM
    (from djbHash to xxh3).

    Though further changes would be needed to cache this value to disk - we
    shouldn't rely on the StringMap::hash remaining the same in the
    future/this value should not be serialized to disk. If we want cache
    this value StringMap should take a hashing template parameter to allow
    for a fixed hash to be requested.

This reverts commit 5bc1adff.
Effectively reapplying the original 2e197602.