c++: this capture in template [PR125384]

The r16-1019 change to special-case 'this' capture handling dropped the
'const' qualifier from the capture proxy, so the call to rvalue in
finish_this_expr no longer built a NOP_EXPR to express dropping that
'const'.

	PR c++/125384
	PR c++/113563

gcc/cp/ChangeLog:

	* lambda.cc (build_capture_proxy): 'this' capture proxy is const.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/lambda/lambda-this23.C: New test.

(cherry picked from commit 4491ee5688d0bcd69956af4126e9e4643d951178)