[cxx-interop] Install `swift/bridging` header in a different location
The recent Clang compiler change makes `-fno-modulemap-allow-subdirectory-search` the default behavior. This means that projects that use C++ interop and `#include <swift/bridging>` no longer compile, since Clang won't search for the SwiftBridging module under `usr/include/swift` anymore.
This change moves the header to `usr/include/SwiftBridging`. The directory name matches the Clang module name, which will help Clang find this header along with the modulemap.
rdar://123334601