Fortran: ALLOCATE of fixed-length CHARACTER with SOURCE/MOLD [PR113793]

Fortran: ALLOCATE of fixed-length CHARACTER with SOURCE/MOLD [PR113793]

F2008 requires for ALLOCATE with SOURCE= or MOLD= specifier that the kind
type parameters of allocate-object and source-expr have the same values.
Add compile-time diagnostics for different character length and a runtime
check (under -fcheck=bounds).  Use length from allocate-object to prevent
heap corruption and to allow string padding or truncation on assignment.

gcc/fortran/ChangeLog:

	PR fortran/113793
	* resolve.cc (resolve_allocate_expr): Reject ALLOCATE with SOURCE=
	or MOLD= specifier for unequal length.
	* trans-stmt.cc (gfc_trans_allocate): If an allocatable character
	variable has fixed length, use it and do not use the source length.
	With bounds-checking enabled, add a runtime check for same length.

gcc/testsuite/ChangeLog:

	PR fortran/113793
	* gfortran.dg/allocate_with_source_29.f90: New test.
	* gfortran.dg/allocate_with_source_30.f90: New test.
	* gfortran.dg/allocate_with_source_31.f90: New test.