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.
The locations stored in .swiftsourceinfo included the presumed file,
line, and column. When a location is requested it would read these, open
the external file, create a line map, and find the offset corresponding
to that line/column.
The offset is known during serialization though, so output it as well to
avoid having to read the file and generate the line map.
Since the serialized location is returned from `Decl::getLoc()`, it
should not be the presumed location. Instead, also output the line
directives so that the presumed location can be built as per normal
locations.
Finally, move the cache out of `Decl` and into `ASTContext`, since very
few declarations will actually have their locations deserialized. Make
sure to actually write to that cache so it's used - the old cache was
never written to.
