Rename StringRef::endswith references to StringRef::ends_with

Missed this when doing the `startswith` renaming. `endswith` has also
been deprecated upstream (and presumably soon to be removed).

lib/Serialization/SerializedModuleLoader.cpp

@@ -1320,7 +1320,7 @@ bool swift::extractCompilerFlagsFromInterface(
       continue;
     auto spelling = ArgSaver.save(parse->getSpelling());
     auto &values = parse->getValues();
-    if (spelling.endswith("=")) {
+    if (spelling.ends_with("=")) {
       // Handle the case like -tbd-install_name=Foo. This should be rare because
       // most equal-separated arguments are alias to the separate form.
       assert(values.size() == 1);