PR c++/88761 - ICE with reference capture of constant.

PR c++/88761 - ICE with reference capture of constant.

Here, we capture nf, then the use of the proxy decays to a constant during
semantic processing of +nf.  Since we saw some decay from proxy to constant,
we walk through the lambda body to see which proxies are still used, but we
weren't walking into subtrees of DECL_EXPR at all, so we missed the use of
&nf in the initializer of y, and removed the capture.  But then at
instantiation time we try to use nf, don't have a proxy anymore, and ICE.

	* lambda.c (mark_const_cap_r): Do walk subtrees of DECL_EXPR for
	non-proxy decls.

From-SVN: r268471