This aligns `ClangDiagnosticManagerAdapter` with how we set up the
diagnostics in `ClangModulesDeclVendor`. We fixed lifetime issues around
the same kind of setup here:
https://github.com/llvm/llvm-project/pull/167724
This class didn't suffer from the same lifetime issue because it used
`shared_ptr`s. So the stream wasn't freed before
`~TextDiagnosticPrinter` accessing it. But that begged the question of
why these are `shared_ptr`s in the first place. This patch makes these
`unique_ptr`s and fixes the destruction order that would now be an
issue.
87da62028e