[cxx-interop] Fix dependency between CxxStdlib and CxxStdlibShim
This is the second part of the fix for `error: no such module 'CxxStdlibShim' when building Swift`.
`add_subdirectory(std)` was executed before `subdirectory(cxxshim)`, which caused `DEPENDS libcxxshim_modulemap` to have no effect.
rdar://108007693