Tests/Module/WCDH: write multi_file_compiler_detection.h before using it

This was broken in commit 98e6d1e5
(Tests/Module/WCDH: make it work with only C features defined) when all C tests
were made accessible even if no C++ features are available, but the header was
only created if C++ features are available. Fix it by creating the header
unconditionally before any checks on the available features.