[clangd] Use starts_with instead of startswith in CompileCommands.cpp (NFC)

llvm-project/clang-tools-extra/clangd/CompileCommands.cpp:324:52:
 error: 'startswith' is deprecated: Use starts_with instead [-Werror,-Wdeprecated-declarations]
  324 |         Cmd, [&](llvm::StringRef Arg) { return Arg.startswith(Flag); });
      |                                                    ^~~~~~~~~~
      |                                                    starts_with