c++: lambda in DMI in class template [PR95870]

c++: lambda in DMI in class template [PR95870]

Here enclosing_instantiation_of was failing to find a match because otctx is
struct S<T> and current_function_decl is S<int>::S(), so the latter has more
function contexts, and we end up trying to compare S() to NULL_TREE.

After spending a bit of time working on establishing the correspondence in
this case (class <=> constructor), it occurred to me that we could just use
DECL_SOURCE_LOCATION, which is unique for lambdas, since they cannot be
redeclared.  Since we're so close to release, for now I'm only doing this
for the case that was failing before.

gcc/cp/ChangeLog:

	PR c++/95870
	* pt.c (enclosing_instantiation_of): Compare DECL_SOURCE_LOCATION if
	there is no enclosing non-lambda function.

gcc/testsuite/ChangeLog:

	PR c++/95870
	* g++.dg/cpp0x/lambda/lambda-nsdmi10.C: New test.