[XRay][tools] Handle "no subcommand" case for llvm-xray

[dberris]

Summary

Currently the llvm-xray commandline tool fails to handle the case for
when no subcommand is provided in a graceful manner. This fixes that to
print the help message explaining the subcommands and the available
options.

[dberris]

@pelikan correctly points out that this is missing a test. Will add that in a bit.

[dberris]

• fixup: add a test

[pelikan]

Thumbs up. Like. +1.

llvm/trunk/test/tools/llvm-xray/X86/no-subcommand-noassert.txt

+; RUN: llvm-xray | FileCheck %s
+
+; CHECK:  OVERVIEW: XRay Tools

llvm/trunk/tools/llvm-xray/llvm-xray.cc

 using namespace llvm::xray;
 
 int main(int argc, char *argv[]) {
   cl::ParseCommandLineOptions(argc, argv,
                               "XRay Tools\n\n"
                               "  This program consolidates multiple XRay trace "
                               "processing tools for convenient access.\n");
   for (auto *SC : cl::getRegisteredSubcommands()) {
-    if (*SC)
+    if (*SC) {
+      // If no subcommand was provided, we need to explicitly check if this is
+      // the top-level subcommand.
+      if (SC == &*cl::TopLevelSubCommand) {
+        cl::PrintHelpMessage(false, true);
+        return 0;
+      }
       if (auto C = dispatch(SC)) {
         ExitOnError("llvm-xray: ")(C());
         return 0;
       }
     }
+  }
 
+  // If all else fails, we still print the usage message.
   cl::PrintHelpMessage(false, true);
 }