[CxxInterop] Change C++ interop header ordering
Consider the case of a toolchain that includes clang is being used to
compile Swift. With the current ordering, the header will be found
relative to that toolchain, which could be out of date as compared to
tip. This is the case today if eg. a 5.7 toolchain is used to build
main/5.9 as the shim header is missing a definition.
