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.

lib/APIDigester/ModuleAnalyzerNodes.cpp

@@ -648,8 +648,8 @@ SDKNode* SDKNode::constructSDKNode(SDKContext &Ctx,
   };
   static auto getAsBool = [&](llvm::yaml::Node *N) -> bool {
     auto txt = cast<llvm::yaml::ScalarNode>(N)->getRawValue();
-    assert(txt.startswith("false") || txt.startswith("true"));
-    return txt.startswith("true");
+    assert(txt.starts_with("false") || txt.starts_with("true"));
+    return txt.starts_with("true");
   };
   SDKNodeKind Kind;
   SDKNodeInitInfo Info(Ctx);