[ADT] Add type-safe methods to FoldingSetImpl; NFC.
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@305241 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm