Sema: move is_convertible outside of class decl

Ensure the class is fully defined before performing the `is_convertible`
check which may trigger a compiler builtin.  Before the class is
complete, the compiler can not technically perform the check on it.
This fixes the build with `cl`.

Author: compnerd

lib/Sema/TypeCheckAccess.cpp

@@ -1494,12 +1494,12 @@ class ImplementationOnlyImportChecker
                               offendingType->getFullName(), M->getName());
       highlightOffendingType(TC, diag, complainRepr);
     }
-
-    static_assert(std::is_convertible<DiagnoseGenerically,
-                                      CheckImplementationOnlyCallback>::value,
-                  "DiagnoseGenerically has wrong call signature");
   };
 
+  static_assert(std::is_convertible<DiagnoseGenerically,
+                                    CheckImplementationOnlyCallback>::value,
+                "DiagnoseGenerically has wrong call signature");
+
   DiagnoseGenerically getDiagnoseCallback(const Decl *D) {
     return DiagnoseGenerically(TC, D);
   }