[Macros] Don't require a declaration of `#externalMacro`.
To enable the declaration and use of macros from a newer Swift
compiler, when the standard library is from an older Swift compiler
that doesn't provide a declaration of `externalMacro`, short-circuit
the type-checking of `#externalMacro(module: "A", type: "B")` when it
is used to declare a new macro.
