IR: Reuse `getImpl()` for `getDistinct()`, NFC

Merge `getDistinct()`'s implementation with those of `get()` and
`getIfExists()` for both `MDTuple` and `MDLocation`.  This will make it
easier to scale to supporting temporaries.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226497 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed