support TARGET_MEM_REF in C/C++ error pretty-printing [PR97197]

support TARGET_MEM_REF in C/C++ error pretty-printing [PR97197]

> See my comment above for Martins attempts to improve things.  I don't
> really want to try decide what to do with those late diagnostic IL
> printing but my commit was blamed for showing target-mem-ref unsupported.
>
> I don't have much time to spend to think what to best print and what not,
> but yes, printing only the MEM_REF part is certainly imprecise.

Here is an updated version of the patch that prints TARGET_MEM_REF the way
it should be printed - as C representation of what it actually means.
Of course it would be better to have the original expressions, but with the
late diagnostics we no longer have them.

2020-10-05  Richard Biener  <rguenther@suse.de>
	    Jakub Jelinek  <jakub@redhat.com>

	PR c++/97197
gcc/cp/
	* error.c (dump_expr): Handle TARGET_MEM_REF.
gcc/c-family/
	* c-pretty-print.c: Include langhooks.h.
	(c_pretty_printer::postfix_expression): Handle TARGET_MEM_REF as
	expression.
	(c_pretty_printer::expression): Handle TARGET_MEM_REF as
	unary_expression.
	(c_pretty_printer::unary_expression): Handle TARGET_MEM_REF.