The diagnostics engine has some code to pretty-print a declaration when
there is no source location for that declaration. The declaration is
pretty-printed into a source buffer, and a source location into that
buffer is synthesizes. This applies to synthesized declarations as well
as those imported from Swift modules (without source code) or from Clang.
Reimplement this pretty-printing for declarations as a request. In
doing so, change the manner in which we do the printing: the
diagnostics engine printed the entire enclosing type into a buffer
whose name was the module + that type. This meant that the buffer was
shared by every member of that type, but also meant that we would end
up deserializing a lot of declarations just for printing and
potentially doing a lot more work for these diagnostics.
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.
