[cxx-interop] Recognize `CxxStdlib` as an alias for `std`
C++ standard library module is called `std`. To make it more clear to a Swift developer that this module is a C++ stdlib, and not a Swift stdlib, let's rename it to `CxxStdlib`.
This is the first step in the module rename. We don't ban `std` in this patch to be able to build SwiftCompilerSources with hosttools until a new Swift compiler is shipped.
