c++: NRV in lambda in template [PR91217]

c++: NRV in lambda in template [PR91217]

tsubst_lambda_expr was producing a function with two blocks that claimed to
be the outermost block in the function body, one from the call to
start_lambda_function in tsubst_lambda_expr, and one from tsubsting the
block added by start_lambda_function when we first parsed the lambda.  This
messed with the named return value optimization, which only works for
variables in the outermost block.

gcc/cp/ChangeLog:

	PR c++/91217
	* pt.c (tsubst_lambda_expr): Skip the body block from
	DECL_SAVED_TREE.

gcc/testsuite/ChangeLog:

	PR c++/91217
	* g++.dg/opt/nrv20.C: New test.