[ADT] Add type-safe methods to FoldingSetImpl; NFC.

Thankfully, this hasn't uncovered any new bugs. With this, issues like
the one fixed in r305207 (hopefully) shouldn't happen again.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305241 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed