fortran: Move the clobber generation code

fortran: Move the clobber generation code

This change inlines the clobber generation code from
gfc_conv_expr_reference to the single caller from where the add_clobber
flag can be true, and removes the add_clobber argument.

What motivates this is the standard making the procedure call a cause
for a variable to become undefined, which translates to a clobber
generation, so clobber generation should be closely related to procedure
call generation, whereas it is rather orthogonal to variable reference
generation.  Thus the generation of the clobber feels more appropriate
in gfc_conv_procedure_call than in gfc_conv_expr_reference.

Behaviour remains unchanged.

gcc/fortran/ChangeLog:

	* trans.h (gfc_conv_expr_reference): Remove add_clobber
	argument.
	* trans-expr.cc (gfc_conv_expr_reference): Ditto. Inline code
	depending on add_clobber and conditions controlling it ...
	(gfc_conv_procedure_call): ... to here.