[CIR] Implement lowering for LinkageSpecDecl (#137634)
Like the NamespaceDecl, this is just emitted as a decl-context, where
all its internal declarations get emitted for it. The incubator doesn't
seem to have any good tests for this, so I wrote what I could think of
as a half-decent test for this one, though the lowering doesn't manage
whether these should be mangled, so the test is mostly just for
spot-checking purposes.
I'm implementing this as it will make writing further tests for future
features a little easier.
