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/ModuleFile.cpp

@@ -289,7 +289,7 @@ Status ModuleFile::associateWithFileContext(FileUnit *file, SourceLoc diagLoc,
   // In Swift 6 mode, we do not inherit search paths from loaded non-SDK modules.
   if (!ctx.LangOpts.isSwiftVersionAtLeast(6) &&
       (SDKPath.empty() ||
-       !Core->ModuleInputBuffer->getBufferIdentifier().startswith(SDKPath))) {
+       !Core->ModuleInputBuffer->getBufferIdentifier().starts_with(SDKPath))) {
     for (const auto &searchPath : Core->SearchPaths) {
       ctx.addSearchPath(
         ctx.SearchPathOpts.SearchPathRemapper.remapPath(searchPath.Path),