[Support]Look up in top-level subcommand as a fallback when looking options for a custom subcommand. (#71776)

**Context:**

- In https://lists.llvm.org/pipermail/llvm-dev/2016-June/101804.html and commit 07670b3, `cl::SubCommand` is introduced.
- Options that don't specify subcommand goes into a special 'top level' subcommand.

**Motivating Use Case:**
- The motivating use case is to refactor `llvm-profdata` to use `cl::SubCommand` to organize subcommands. See
#71328. A valid use case that's not supported before this patch is shown below

```
  // show-option{1,2} are associated with 'show' subcommand.
  // top-level-option3 is in top-level subcomand (e.g., `profile-isfs` in SampleProfReader.cpp)
  llvm-profdata show --show-option1 --show-option2 --top-level-option3
```

- Before this patch, option handler look-up will fail with the following error message "Unknown command line argument --top-level-option3".
- After this patch, option handler look-up will look up in sub-command options first, and use top-level subcommand as a fallback, so 'top-level-option3' is parsed correctly.

Author: mingmingl-llvm

llvm/lib/Support/CommandLine.cpp

@@ -1667,6 +1667,13 @@ bool CommandLineParser::ParseCommandLineOptions(int argc,
       Handler = LookupLongOption(*ChosenSubCommand, ArgName, Value,
                                  LongOptionsUseDoubleDash, HaveDoubleDash);
 
+      // If Handler is not found in a specialized subcommand, look up handler
+      // in the top-level subcommand.
+      // cl::opt without cl::sub belongs to top-level subcommand.
+      if (!Handler && ChosenSubCommand != &SubCommand::getTopLevel())
+        Handler = LookupLongOption(SubCommand::getTopLevel(), ArgName, Value,
+                                   LongOptionsUseDoubleDash, HaveDoubleDash);
+
       // Check to see if this "option" is really a prefixed or grouped argument.
       if (!Handler && !(LongOptionsUseDoubleDash && HaveDoubleDash))
         Handler = HandlePrefixedOrGroupedOption(ArgName, Value, ErrorParsing,

llvm/unittests/Support/CommandLineTest.cpp

@@ -525,6 +525,59 @@ TEST(CommandLineTest, LookupFailsInWrongSubCommand) {
   EXPECT_FALSE(Errs.empty());
 }
 
+TEST(CommandLineTest, TopLevelOptInSubcommand) {
+  enum LiteralOptionEnum {
+    foo,
+    bar,
+    baz,
+  };
+
+  cl::ResetCommandLineParser();
+
+  // This is a top-level option and not associated with a subcommand.
+  // A command line using subcommand should parse both subcommand options and
+  // top-level options.  A valid use case is that users of llvm command line
+  // tools should be able to specify top-level options defined in any library.
+  cl::opt<std::string> TopLevelOpt("str", cl::init("txt"),
+                                   cl::desc("A top-level option."));
+
+  StackSubCommand SC("sc", "Subcommand");
+  StackOption<std::string> PositionalOpt(
+      cl::Positional, cl::desc("positional argument test coverage"),
+      cl::sub(SC));
+  StackOption<LiteralOptionEnum> LiteralOpt(
+      cl::desc("literal argument test coverage"), cl::sub(SC), cl::init(bar),
+      cl::values(clEnumVal(foo, "foo"), clEnumVal(bar, "bar"),
+                 clEnumVal(baz, "baz")));
+  StackOption<bool> EnableOpt("enable", cl::sub(SC), cl::init(false));
+  StackOption<int> ThresholdOpt("threshold", cl::sub(SC), cl::init(1));
+
+  const char *PositionalOptVal = "input-file";
+  const char *args[] = {"prog",    "sc",        PositionalOptVal,
+                        "-enable", "--str=csv", "--threshold=2"};
+
+  // cl::ParseCommandLineOptions returns true on success. Otherwise, it will
+  // print the error message to stderr and exit in this setting (`Errs` ostream
+  // is not set).
+  ASSERT_TRUE(cl::ParseCommandLineOptions(sizeof(args) / sizeof(args[0]), args,
+                                          StringRef()));
+  EXPECT_STREQ(PositionalOpt.getValue().c_str(), PositionalOptVal);
+  EXPECT_TRUE(EnableOpt);
+  // Tests that the value of `str` option is `csv` as specified.
+  EXPECT_STREQ(TopLevelOpt.getValue().c_str(), "csv");
+  EXPECT_EQ(ThresholdOpt, 2);
+
+  for (auto &[LiteralOptVal, WantLiteralOpt] :
+       {std::pair{"--bar", bar}, {"--foo", foo}, {"--baz", baz}}) {
+    const char *args[] = {"prog", "sc", LiteralOptVal};
+    ASSERT_TRUE(cl::ParseCommandLineOptions(sizeof(args) / sizeof(args[0]),
+                                            args, StringRef()));
+
+    // Tests that literal options are parsed correctly.
+    EXPECT_EQ(LiteralOpt, WantLiteralOpt);
+  }
+}
+
 TEST(CommandLineTest, AddToAllSubCommands) {
   cl::ResetCommandLineParser();