`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
Calling `getInnermostDeclContext()->getParentSourceFile()` on a macro-produced decl does not seem to be a reliable way to obtain the macro expansion source file, because `PatternBindingDecl` is not a `DeclContext` and `getInnermostDeclContext()` falls back outside the macro expansion file. This patch switches to using `getSourceFileContainingLocation` when possible.
Resolves rdar://109376568.
