Fortran: f_c_string intrinsic improvements

Fortran: f_c_string intrinsic improvements

The existing implementation of f_c_string is quite inefficient, doing
either 2 or 3 allocations and copies of the input string prefix.  This
rewrite adds folding for constant string arguments and handles other
cases with a single allocation and copy.

This patch also adds the missing documentation for this intrinsic to the
gfortran manual.

gcc/fortran/ChangeLog
	* intrinsic.texi (F_C_STRING): New section.
	* trans-intrinsic.cc (conv_trim): Delete.
	(conv_isocbinding_function): Rewrite the F_C_STRING case.

gcc/testsuite/ChangeLog
	* gfortran.dg/f_c_string3.f90: New.
	* gfortran.dg/f_c_string4.f90: New.
	* gfortran.dg/f_c_string5.f90: New.