Clang importer: when dropping the variadic parameter, drop the variadic parameter.

Fixes iOS build.

Author: DougGregor

lib/ClangImporter/ClangImporter.cpp

@@ -2198,7 +2198,7 @@ auto ClangImporter::Implementation::importFullName(
     if (objcMethod->isVariadic() && shouldMakeSelectorNonVariadic(selector)) {
       --numArgs;
       result.DroppedVariadic = true;
-      params = params.slice(1);
+      params = params.drop_back(1);
     }
 
     for (unsigned index = 0; index != numArgs; ++index) {

lib/ClangImporter/ImportDecl.cpp

@@ -3162,7 +3162,7 @@ namespace {
 
       // If we dropped the variadic, handle it now.
       if (importedName.DroppedVariadic) {
-        params = params.slice(1);
+        params = params.drop_back(1);
         variadic = false;
       }