c++: Improve error recovery for =.

In a template we were happily embedding error_mark_node in a MODOP_EXPR,
leading to confusion later.

gcc/cp/ChangeLog:

	* typeck.c (build_x_modify_expr): Handle error_mark_node arguments.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/lambda/lambda-ice30.C: Adjust.
	* g++.dg/cpp0x/lambda/lambda-ice31.C: Adjust.
	* g++.dg/ext/fixed1.C: Adjust.
	* g++.dg/template/crash107.C: Adjust.
	* g++.dg/template/error35.C: Adjust.
	* g++.dg/template/sizeof-template-argument.C: Adjust.