[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 07670b3e984db32f291373fe12c392959f2aff67, `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
https://github.com/llvm/llvm-project/pull/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.

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Support/CommandLine.cpp b/llvm/lib/Support/CommandLine.cpp
index 55633d7..a7e0cae 100644
--- a/llvm/lib/Support/CommandLine.cpp
+++ b/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,