Optionally enable this infrastructure through an undocumented
`CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP` variable.  Currently this is
experimental and intended for use by compiler writers to implement their
scanning tools.  Warn as such when the feature is activated.  Later when
compilers provide the needed scanning tools we can enable this variable
from our corresponding compiler information modules.  It is never meant
to be set by project code.

When enabled, generate a build graph similar to what we use for Fortran
module dependencies.  There are some differences needed because we can
scan dependencies without explicit preprocessing, and can directly
compile the original source afterward.

Co-Author: Ben Boeckel <ben.boeckel@kitware.com>