c++: lvalueness of non-dependent assignment expr [PR114994]

c++: lvalueness of non-dependent assignment expr [PR114994]

r14-4111-g6e92a6a2a72d3b made us check non-dependent simple assignment
expressions ahead of time and give them a type, as was already done for
compound assignments.  Unlike for compound assignments however, if a
simple assignment resolves to an operator overload we represent it as a
(typed) MODOP_EXPR instead of a CALL_EXPR to the selected overload.
(I reckoned this was at worst a pessimization -- we'll just have to repeat
overload resolution at instantiatiation time.)

But this turns out to break the below testcase ultimately because
MODOP_EXPR (of non-reference type) is always treated as an lvalue
according to lvalue_kind, which is incorrect for the MODOP_EXPR
representing x=42.

We can fix this by representing such class assignment expressions as
CALL_EXPRs as well, but this turns out to require some tweaking of our
-Wparentheses warning logic and may introduce other fallout making it
unsuitable for backporting.

So this patch instead fixes lvalue_kind to consider the type of a
MODOP_EXPR representing a class assignment.

	PR c++/114994

gcc/cp/ChangeLog:

	* tree.cc (lvalue_kind) <case MODOP_EXPR>: For a class
	assignment, consider the result type.

gcc/testsuite/ChangeLog:

	* g++.dg/template/non-dependent32.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>