[cxx-import] a Swift module that imports 'CxxStdlib' and forces the client to enable interop produces unclear diagnostic

[hyp]

RIght now when a Swift module A enables interoperability and imports Cxxstdlib, and Swift module B that consumes it doesn't, the compiler will fail to compile B with the following error:

<unknown>:0: error: cannot load underlying module for 'CxxStdlib'

This diagnostic should be improved, and should suggest to the user to enable C++ interoperability.