Emitting Swift diagnostics in Clang buffers requires making those
buffers valid places to put Swift SourceLocs, which means making a
mirror of those buffers in the Swift SourceManager. This isn't a copy;
instead, any Clang SourceManagers that are involved are kept alive
until the importer is torn down. (There might be more than one because
of diagnostics emitted during module building.)
For a long time we only emitted diagnostics in Clang buffers if the
diagnostics came from Clang, but then we added another case for custom
Swift names that fail to import. I'm about to add another such
diagnostic, so let's formalize this buffer mapping first.
No intended functionality change.
