[clang] [clang-repl] adding documentation and information for %commands (PR #150348)

[David Spickett via cfe-commits]

================
@@ -364,15 +366,34 @@ int main(int argc, const char **argv) {
       }
 
       Input += L;
+      // If we add more % commands, there should be better architecture than
+      // this.
       if (Input == R"(%quit)") {
         break;
       }
       if (Input == R"(%undo)") {
         if (auto Err = Interp->Undo())
           llvm::logAllUnhandledErrors(std::move(Err), llvm::errs(), "error: ");
+      } else if (Input == R"(%help)") {
+        OS << "%help\t\tlist clang-repl %commands\n"
----------------
DavidSpickett wrote:

Since OS is only used once you can simplify to:
```
llvm::outs() << "...."
```

https://github.com/llvm/llvm-project/pull/150348