Add get_{fun,const,type} methods to ShallowDeclProvider
Summary: Currently, decls of toplevel functions, global constants, and type aliases are inaccessible: they are stored in a ShallowDeclProvider's backing cache, but the ShallowDeclProvider interface doesn't provide a means of reading them (it only provides accessors for class decls). This diff adds APIs for fetching non-class decls.
Reviewed By: shayne-fletcher
Differential Revision:
D34406673
fbshipit-source-id:
633a2ab3a29fbd72bb8b08eb0033b68086d1537c