[Macros] Resolve in-process macros via external type name.
This approach works for both macros built into the compiler (e.g., the
builtin macros) as well as those that are loaded via plugin but don't
conform to the _CompilerPluginSupport protocol.
This eliminates all uses of the `MacroSystem` itself in ASTGen, and
pushes more of the implementation through ASTGen.
