DebugInfo: Introduce DIBuilder::replaceTemporary()

Add `DIBuilder::replaceTemporary()` as a replacement for
`DIDescriptor::replaceAllUsesWith()`.  I'll update clang to use the new
method, and then come back to delete the original.

This method dispatches to `replaceAllUsesWith()` or
`replaceWithUniqued()`, depending on whether the replacement is actually
a different node from the original.

llvm-svn: 234695