Write `MDLocation::getInlinedAtScope()` and use it to re-implement
`DebugLoc::getScopeNode()` (and simplify `DISubprogram::Verify()`).
This follows the inlined-at linked list and returns the scope of the
deepest/last location.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233568 91177308-0d34-0410-b5e6-96231b3b80d8