PR c++/88613 - ICE with use of const var in lambda.

PR c++/88613 - ICE with use of const var in lambda.

The issue here was that we were cp_folding a location wrapper around a
lambda capture proxy before it had been mark_rvalue_used.  I considered
adding mark_rvalue_use calls to build_new_op_1, but it seems appropriate to
have them in cp_fold_maybe_rvalue when we know we're trying to produce an
rvalue.

The change to mark_use is for a related issue: when we change the operand of
the location wrapper from VAR_DECL to INTEGER_CST, we need the TREE_CODE of
the location wrapper to change as well, from VIEW_CONVERT_EXPR to
NON_LVALUE_EXPR.

	* expr.c (mark_use): Fix location wrapper handling.
	* cp-gimplify.c (cp_fold_maybe_rvalue): Call mark_rvalue_use.

From-SVN: r267859