We should no longer say "macro implementation must be provided via '-load-plugin-library'" as there are multiple ways to pick up a macro dependency. Here's an improved version:
> warning: external macro implementation type 'MissingModule.MissingType' could not be found for macro 'missingMacro1'; the type must be public and provided by a macro target in a Swift package, or via '-plugin-path' or '-load-plugin-library'
The macro tests were all using "REQUIRES: OS=macosx" as a proxy for
"have the Swift Swift parser". There was an existing feature for this,
but it was just checking whether the path was passed through. Fix that
to use the same variable as in CMake.
Also remove all extraneous `-I` and `-L` to the host libs in the target
invocations.
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.
