DebugInfo: Use TempMDNode in DIDescriptor::replaceAllUsesWith()

DebugInfo: Use TempMDNode in DIDescriptor::replaceAllUsesWith()

Start using `TempMDNode` in `DIDescriptor::replaceAllUsesWith()`
(effectively `std::unique_ptr<MDNode, MDNode::deleteTemporary>`).

Besides making ownership more explicit, this prepares for when
`DIDescriptor` refers to nodes that are *not* `MDTuple`.  The old logic
for "replacing" a node with itself used `MDNode::get()` to return a new
(uniqued) `MDTuple`, while the new logic just defers to
`MDNode::replaceWithUniqued()` (which also typically saves an allocation
and RAUW traffic by mutating the temporary in place).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230879 91177308-0d34-0410-b5e6-96231b3b80d8