Eventually we should just have one IndexSymbol, with all the
information. Once we kill FuncDeclIndexSymbol and can get rid of the
enum, we will regain most of the bytes we lost by inlining receiverUSR.
As a first step toward having more detailed "role" information, thread
through a SymbolRole bitset. For now it just contains the existing ref
vs. definition status.
Having a single interface for source files and modules and having to
pass in a buffer id was a crappy API. This splits the interface to take
either a Module or a SourceFile and handles the buffer id stuff
internally.
