[Pitch] Change CChar32 to UInt32

Hello! I'd like to propose a change to CChar32, Swift's C/C++ type alias for char32_t. char32_t can hold values that are not valid Unicode scalars, so importing it as Unicode.Scalar is unsound. This is technically a breaking change for Swift code that relies on the current unsound mapping, but I believe there will be few actual breakages in practice. Summary of changes Change CChar32, the Swift type for C++ char32_t, from Unicode.Scalar to UInt32. Motivation Swift's Unicode.Scalar is always a...