[C++] Fix double pointers to foreign ref types
When a C++ type `ForeignTy` is imported as a foreign reference type, Swift should strip a level of indirection from parameters of that type, so e.g. `ForeignTy *` and `ForeignTy &` should be imported to Swift as just `ForeignTy`. However, it should only strip *one* level of indirection. importType() was instead stripping *all* levels, so things like `ForeignTy **` and `ForeignTy *&` were *also* being incorrectly imported as `ForeignTy`. Narrow this behavior so it only applies to a single level of indirection and add a few test cases to pin down the specifics.
Fixes rdar://123905345.
