PR c++/56973, DR 696 - capture constant variables only as needed.

PR c++/56973, DR 696 - capture constant variables only as needed.

	* expr.c (mark_use): Split out from mark_rvalue_use and
	mark_lvalue_use.  Handle lambda capture of constant variables.
	(mark_lvalue_use_nonread): New.
	* semantics.c (process_outer_var_ref): Don't capture a constant
	variable until forced.
	* pt.c (processing_nonlambda_template): New.
	* call.c (build_this): Check it.
	* decl2.c (grok_array_decl): Call mark_rvalue_use and
	mark_lvalue_use_nonread.
	* init.c (constant_value_1): Don't call mark_rvalue_use.
	* typeck.c (build_static_cast): Handle lambda capture.

From-SVN: r253266