PR fortran/93340 - fix missed substring simplifications

PR fortran/93340 - fix missed substring simplifications

Substrings were not reduced early enough for use in initializations,
such as DATA statements.  Add an early simplification for substrings
with constant starting and ending points.

gcc/fortran/ChangeLog:

	* gfortran.h (gfc_resolve_substring): Add prototype.
	* primary.c (match_string_constant): Simplify substrings with
	constant starting and ending points.
	* resolve.c: Rename resolve_substring to gfc_resolve_substring.
	(gfc_resolve_ref): Use renamed function gfc_resolve_substring.

gcc/testsuite/ChangeLog:

	* substr_10.f90: New test.
	* substr_9.f90: New test.