swift234

Serialization: Fix a couple of issues with serializing context params of generic SILFunctions.

- Parameterize maybeReadGenericParams' BitstreamCursor so that we can read from the correct cursor when trying to read the generic params of a SILFunction.
- Only serialize the context generic params for SILFunctions for which we're serializing a complete definition. This fixes issues with us getting the wrong archetypes forward-declared from references in other modules.

This gets me a clean build when applied against r13984.

Swift SVN r14005