Rename startswith to starts_with

LLVM is presumably moving towards `std::string_view` -
`StringRef::startswith` is deprecated on tip. `SmallString::startswith`
was just renamed there (maybe with some small deprecation inbetween, but
if so, we've missed it).

The `SmallString::startswith` references were moved to
`.str().starts_with()`, rather than adding the `starts_with` on
`stable/20230725` as we only had a few of them. Open to switching that
over if anyone feels strongly though.

tools/swift-ast-script/ASTScriptConfiguration.cpp

@@ -63,7 +63,7 @@ ASTScriptConfiguration::parse(CompilerInstance &compiler,
   // Parse the arguments.
   while (!hadError && !args.empty()) {
     StringRef arg = popArg(nullptr);
-    if (!arg.startswith("-")) {
+    if (!arg.starts_with("-")) {
       setScriptFile(arg);
     } else if (arg == "-f") {
       StringRef filename = popArg("expected path after -f");