Properly parse invariant &MEM addresses in the GIMPLE FE

Properly parse invariant &MEM addresses in the GIMPLE FE

Currently the frontend rejects those addresses as not lvalues
because the C frontend doens't expect MEM_REF or TARGET_MEM_REF
to appear (but they would be valid lvalues there).  The following
fixes that by amending lvalue_p.

The change also makes the dumping of the source of the testcase
valid for the GIMPLE FE by not eliding the '&' when dumping
string literals.

2021-10-06  Richard Biener  <rguenther@suse.de>

gcc/c/
	* c-typeck.c (lvalue_p): Also allow MEM_REF and TARGET_MEM_REF.

gcc/
	* tree-pretty-print.c (dump_generic_node): Do not elide
	printing '&' when dumping with -gimple.

gcc/testsuite/
	* gcc.dg/gimplefe-47.c: New testcase.