[Metadata][NFC] Make MDNode::resolve() public in preparation for the fix to PR33930.
[Metadata][NFC] Make MDNode::resolve() public in preparation for the fix to PR33930.

Reviewers: aprantl

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@317018 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit c6f644f195b7571df6849ed385feac2940cc48bf)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm