Introduced DeclContext::isDependentContext, which determines whether a

given DeclContext is dependent on type parameters. Use this to
properly determine whether a TagDecl is dependent; previously, we were
missing the case where the TagDecl is a local class of a member
function of a class template (phew!).

Also, make sure that, when we instantiate declarations within a member
function of a class template (or a function template, eventually),
that we add those declarations to the "instantiated locals" map so
that they can be found when instantiating declaration references.

Unfortunately, I was not able to write a useful test for this change,
although the assert() that fires when uncommenting the FIXME'd line in
test/SemaTemplate/instantiate-declref.cpp tells the "experienced user"
that we're now doing the right thing.

llvm-svn: 72526