Change `DICompileUnit::replaceSubprograms()` and
`DICompileUnit::replaceGlobalVariables()` to match the `MDCompileUnit`
equivalents that they're wrapping.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234852 91177308-0d34-0410-b5e6-96231b3b80d8