c++: Give more expressions locations.

c++: Give more expressions locations.

In the testcase for PR94186, we have a SCOPE_REF with no location even
though at one point it was in a cp_expr which had a location.  So let's make
the cp_expr constructor that takes a location apply it to the expression
when possible.

gcc/cp/ChangeLog
2020-03-24  Jason Merrill  <jason@redhat.com>

	* cp-tree.h (cp_expr): When constructing from an expr and a
	location, call protected_set_expr_location.