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/Serialization/ModuleFileSharedCore.cpp

@@ -373,7 +373,7 @@ static ValidationInfo validateControlBlock(
       // recommended configuration and may lead to unreadable swiftmodules.
       StringRef moduleSDK = blobData;
       if (!moduleSDK.empty() && !requiredSDK.empty() &&
-          !requiredSDK.startswith(moduleSDK)) {
+          !requiredSDK.starts_with(moduleSDK)) {
         result.status = Status::SDKMismatch;
         return result;
       }
@@ -571,7 +571,7 @@ std::string serialization::StatusToString(Status S) {
 bool serialization::isSerializedAST(StringRef data) {
   StringRef signatureStr(reinterpret_cast<const char *>(SWIFTMODULE_SIGNATURE),
                          std::size(SWIFTMODULE_SIGNATURE));
-  return data.startswith(signatureStr);
+  return data.starts_with(signatureStr);
 }
 
 ValidationInfo serialization::validateSerializedAST(