Fortran: Fix PR 47485.

The -MT and -MQ options should replace the default target in the
generated dependency file. deps_add_target needs to be called before
cpp_read_main_file, otherwise the original object name is added.

Contributed by Vincent Vanlaer <vincenttc@volkihar.be>

	PR fortran/47485

gcc/fortran/ChangeLog:

	* cpp.cc: fix -MT/-MQ adding additional target instead of
	replacing the default.

gcc/testsuite/ChangeLog:

	* gfortran.dg/dependency_generation_1.f90: New test.

Signed-off-by: Vincent Vanlaer <vincenttc@volkihar.be>