Use real types instead of "Self" for the IR value names of local type data.

Since that's somewhat expensive, allow the generation of meaningful
IR value names to be efficiently controlled in IRGen.  By default,
enable meaningful value names only when generating .ll output.

I considered giving protocol witness tables the name T:Protocol
instead of T.Protocol, but decided that I didn't want to update that
many test cases.

lib/Frontend/CompilerInvocation.cpp

@@ -1125,6 +1125,13 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     Opts.LinkLibraries.push_back(LinkLibrary(A->getValue(), Kind));
   }
 
+  if (auto valueNames = Args.getLastArg(OPT_disable_llvm_value_names,
+                                        OPT_enable_llvm_value_names)) {
+    Opts.HasValueNamesSetting = true;
+    Opts.ValueNames =
+      valueNames->getOption().matches(OPT_enable_llvm_value_names);
+  }
+
   Opts.DisableLLVMOptzns |= Args.hasArg(OPT_disable_llvm_optzns);
   Opts.DisableLLVMARCOpts |= Args.hasArg(OPT_disable_llvm_arc_opts);
   Opts.DisableLLVMSLPVectorizer |= Args.hasArg(OPT_disable_llvm_slp_vectorizer);