[c-interop] Check interface C compatibility only for @_extern(c)

`@_extern(wasm)` has no limitation on the function interface, so the
check should be applied only for `@_extern(c)`.

Author: kateinoigakukun

lib/Sema/TypeCheckAttr.cpp

@@ -2135,16 +2135,18 @@ void AttributeChecker::visitExternAttr(ExternAttr *attr) {
       if (cName->empty())
         diagnose(attr->getLocation(), diag::extern_empty_c_name);
     }
+
+    // Ensure the decl has C compatible interface. Otherwise it produces diagnostics.
+    if (!isCCompatibleFuncDecl(FD)) {
+      attr->setInvalid();
+      // Mark the decl itself invalid not to require body even with invalid ExternAttr.
+      FD->setInvalid();
+    }
   }
 
   // @_cdecl cannot be mixed with @_extern since @_cdecl is for definitions
   if (D->getAttrs().hasAttribute<CDeclAttr>())
     diagnose(attr->getLocation(), diag::extern_only_non_other_attr, "@_cdecl");
-
-  if (!isCCompatibleFuncDecl(FD)) {
-    attr->setInvalid();
-    FD->setInvalid();
-  }
 }
 
 void AttributeChecker::visitUsedAttr(UsedAttr *attr) {

test/attr/attr_extern.swift

@@ -86,8 +86,22 @@ func mixedAttrs_C_Wasm()
 class NonC {}
 @_extern(c)
 func nonCReturnTypes() -> NonC // expected-error {{'NonC' cannot be represented in C}}
+// @_extern(wasm) have no interface limitation
+@_extern(wasm, module: "non-c", name: "return_wasm")
+func nonCReturnTypesWasm() -> NonC
+@_extern(c)
+@_extern(wasm, module: "non-c", name: "return_mixed")
+func nonCReturnTypesMixed() -> NonC // expected-error {{'NonC' cannot be represented in C}}
+
 @_extern(c)
 func nonCParamTypes(_: Int, _: NonC) // expected-error {{'NonC' cannot be represented in C}}
+@_extern(wasm, module: "non-c", name: "param_wasm")
+func nonCParamTypesWasm(_: Int, _: NonC)
+
+@_extern(c)
+@_extern(wasm, module: "non-c", name: "param_mixed")
+func nonCParamTypesMixed(_: Int, _: NonC) // expected-error {{'NonC' cannot be represented in C}}
+
 
 @_extern(c)
 func asyncFuncC() async // expected-error {{async functions cannot be represented in C}}