Revert "[Support] Promote cl::StringSaver to a separate utility"

This reverts commit r215784 / 3f8a26f6fe16cc76c98ab21db2c600bd7defbbaa.

LLD has 3 StringSaver's, one of which takes a lock when saving the
string... Need to investigate more closely.
Revert "Update for LLVM change (StringSaver)"

This reverts commit r215785 / 170ebf4f19459ae51a9561d0e65c87ee4c9b2c97.

LLD has some StringSavers that need to be updated. One of which takes a
lock and I need to investigate that more closely.