style(tvix): Replace remaining typedefs with using statements

Change (mostly) created via:

  fd . -e hh -e cc | parallel clang-tidy -p ~/builds/nix --fix "'--checks=-*,modernize-use-using'"

Change-Id: I90ad6e64a5399f2005dfb9956074ec51c2c78cea
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1830
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libstore/serve-protocol.hh

@@ -9,7 +9,7 @@ namespace nix {
 #define GET_PROTOCOL_MAJOR(x) ((x)&0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x)&0x00ff)
 
-typedef enum {
+using ServeCommand = enum {
   cmdQueryValidPaths = 1,
   cmdQueryPathInfos = 2,
   cmdDumpStorePath = 3,
@@ -19,6 +19,6 @@ typedef enum {
   cmdQueryClosure = 7,
   cmdBuildDerivation = 8,
   cmdAddToStoreNar = 9,
-} ServeCommand;
+};
 
 }  // namespace nix