C++ interop support for multiple C++ runtimes

[tristanlabelle]

C++ interop should support both code that uses the msvc++ runtime and code that uses libc++.

This is accomplished by entering a special libc++ import context when loading a set of specified Clang modules that depend on libc++ instead of msvc STL.

• [x] preload 'ucrt' without libc++ context when transitioning into libc++ context.
• [x] rename the imported 'std' modules in the imported libc++ module map.
• [x] rename the imported 'std' namespace in the imported libc++.
• [x] NOT NEEDED: resolve the decl conflicts between imported libc++ and msvc STL.
• [x] adjust the import paths correctly when transitioning into libc++ context.
• [x] NOT NEEDED: reset the import paths when leaving the libc++ context.
• [x] Verify that libc++ module reference don't pass through the Swift module boundary

[hyp]

Working on finalizing it and putting up the PR.

[hyp]

https://github.com/apple/swift/pull/72843