Move the "current scope" state from the Parser into Action. This

allows Sema some limited access to the current scope, which we only
use in one way: when Sema is performing some kind of declaration that
is not directly driven by the parser (e.g., due to template
instantiatio or lazy declaration of a member), we can find the Scope
associated with a DeclContext, if that DeclContext is still in the
process of being parsed. 

Use this to make the implicit declaration of special member functions
in a C++ class more "scope-less", rather than using the NULL Scope hack.

llvm-svn: 107491