Richard Wei 7502f3ce83 [Macros] Fix existential diagnostics for declaration macro expansions ...

`TypeChecker::checkExistentialTypes` has decl-specific logic and backs out without invoking `ExistentialTypeVisitor` on `TypeDecl`s, but `MacroExpansionDecl` isn't a type decl and would fall into `ExistentialTypeVisitor` which will visit its expansion. The caller of `checkExistentialTypes` already visits auxiliary decls, so here we should only visit arguments and generic arguments of a macro.

Fixes a case where a declaration macro produces a type that conforms to a PAT. We now also run existential diagnostics on generic arguments on a `MacroExpansionDecl` that was previously not handled.

Note: I tried bailing out in `walkToDeclPre` but we should really visit macro arguments and generic arguments. Not walking expansions in `ExistentialTypeVisitor` is also not the right fix because we need to be able to visit macro expansions in arguments.

rdar://109779928

2023-05-25 23:25:58 -07:00

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