DebugInfo: Stop leaking temporaries in DIBuilder::createCompileUnit()

Stop leaking temporary nodes from `DIBuilder::createCompileUnit()`.
`replaceAllUsesWith()` doesn't delete the nodes, so we need to delete
them "manually" (well, `TempMDTuple` does that for us).

Similarly, stop leaking the temporary nodes used for variables of
subprograms.

llvm-svn: 234617