Properly parse the 'default' and 'delete' keywords.

They are actually grammatically considered definitions and parsed
accordingly.

This fixes the outstanding bugs regarding defaulting functions after
their declarations.

We now really nicely diagnose the following construct (try it!)

int foo() = delete, bar;

Still todo: Defaulted functions other than default constructors
            Test cases (including for the above construct)

llvm-svn: 131228