swift234

Sema: Split off openFunctionType() from openType()

- Change openGeneric() to take two DeclContexts, one is the generic
  context for the signature and the second is the generic context
  containing the declaration being opened

- This allows us to clean up the logic around skipProtocolSelfRequirement;
  instead of testing both the DeclContext and its parent, we know
  exactly what DeclContext to test. Also, use the right Self type here,
  instead of always using (0, 0)

- Now that we have the right DeclContexts handy, we can move the
  getGenericTypeContextDepth() call into openGeneric(), simplifying
  callers

- Now that openType() no longer opens generic signatures, it takes fewer
  parameters