fortran: Fix bogus warning with -cpp -fpreprocessed [PR92613]

fortran: Fix bogus warning with -cpp -fpreprocessed [PR92613]

With -fpreprocessed, gfortran skips the C preprocessor, avoiding
spurious diagnostics about characters in Fortran comments.

	PR fortran/92613

gcc/fortran/ChangeLog:

	* cpp.cc (gfc_cpp_preprocess): Skip libcpp for -fpreprocessed.
	Error out for -fpreprocessed -E.
	* f95-lang.cc (gfc_init): Skip libcpp init for -fpreprocessed.
	* scanner.cc (gfc_new_file): Load source directly if preprocessed.
	* invoke.texi (Preprocessing Options): Document -fpreprocessed.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr92613.f90: New test.
	* gfortran.dg/pr92613_2.f90: New test for -E error.

Signed-off-by: Christopher Albert <albert@tugraz.at>