C: Drop qualifiers of assignment expressions. [PR98047]

ISO C17 6.5.15.1 specifies that the result is the
type the LHS would have after lvalue conversion.

2020-12-16  Martin Uecker  <muecker@gwdg.de>

gcc/c/
	PR c/98047
	* c-typeck.c (build_modify_expr): Drop qualifiers.

gcc/testsuite/
	PR c/98047
	* gcc.dg/qual-assign-7.c: New test.