We were calculating these for every decl-occurrence, but we really only need them once per decl.
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.
Leaving only the SourceKit-specific code (e.g. UID generation) behind in SourceKit. rdar://problem/22348041
