[Macros] A few bug fixes for extension macros.

[hborla]

This fixes 3 small issues with extension macros:

1. When resolving extension macro protocol conformances, handle the case where a protocol has a null type repr because it was deserialized. This fixes an issue where imported extension macros with a conformances: list appeared to not add any conformances, and expansion was always passed an empty protocols array.
2. In SILSymbolVisitor, skip local conformance entries from pre-expanded macros. This fixes an assertion failure in TBDGen due to duplicate conformances.
3. Fix a silly copy-paste error in ASTGen that caused a bogus conformance list to be passed to extension macro expansion on the plugin server path.

These changes allow @Observable to adopt extension macros in #66993

[hborla]

@swift-ci please smoke test