[sema] Revise `getCurrentMangleNumberContext` interface. NFC.

- Prefer returning mulitple values using a tuple instead of
  additional pointers/references.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374274 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed