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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215790 91177308-0d34-0410-b5e6-96231b3b80d8