[clang] [clang][ASTImporter] Remove 'hasReturnTypeDeclaredInside' (PR #169504)

[Donát Nagy via cfe-commits]

================
@@ -4073,8 +3851,7 @@ ExpectedDecl ASTNodeImporter::VisitFunctionDecl(FunctionDecl *D) {
     // with a simplified return type.
     // Reuse this approach for auto return types declared as typenames from
     // template params, tracked in FindFunctionDeclImportCycle.
----------------
NagyDonat wrote:

This comment block reflects the old situation by first describing `hasReturnTypeDeclaredInside` and then mentioning that this approach is "reused" to handle other cases which are tracked in `FindFunctionDeclImportCycle`.

Now that everything is handled by `FindFunctionDeclImportCycle`, replace this comment block with an unified explanation.

https://github.com/llvm/llvm-project/pull/169504