[ClangImporter] Extract common import parameter code. Fix Obj-C++ params.

Extract the code of import parameter list into an independent method.
Also merge the differences in code from import method parameters into
it. Both importing function parameters and import method parameters use
the same code to transform parameters, so the improvements done for
functions to support C++ parameters will apply to Obj-C++ methods.

Also extract the creation of parameter info, which is also shared, but
cannot live together in the previous extracted function because methods
perform some checks in between the two parts that functions cannot do.

Add a test that checks the corrected of importing C++ types from Obj-C++
methods. Before these changes the parameters would have been imported as
`UnsafePointer`, but they are now imported as its C++ type (the same as
when they are imported in a C++ function). There are several FIXME in
the test because the parameters are not imported as `@in` in the
initializer, and there are discussions about those `@in` needing to be
`@in_guaranteed` instead.

Author: drodriguez