cmNinjaTargetGenerator: Fix scan rule PREPROCESSED_SOURCE placeholder
When running the module dependencies scan tool for for a language that
does not compile the preprocessed output, we do not actually put the
preprocessed output in the build graph. However, the value of
CMAKE_EXPERIMENTAL_<LANG>_SCANDEP_SOURCE
may reference the placeholder
for the preprocessed source. Populate the placeholder to keep the file
out of the way. In particular, do not clobber the .ddi
file.