When emitting a diagnostic that references a declaration that does not
itself have a source location (e.g., because it was synthesized or
deserialized), the diagnostics engine pretty-prints the declaration
into a buffer so it can provide caret diagnostics pointing to that
declaration.
Start marking those buffers as "generated source buffers", so that we
emit their contents into serialized diagnostics files. This will allow
tools that make use of serialized diagnostics to also show caret
information.
The directory currently seems to have a mix of
tests for import resolution and name lookup.
Therefore split it into two directories;
ImportResolution and NameLookup.
