[Option] Fix simple subcommand with positional arguments (#166859)

Fix subcommand detection when subcommand used with positional arguments.
When there is only one valid subcommand passed,
`ArgList::getSubCommand()` should return the correct subcommand even
there are other positionals passed.

llvm/lib/Option/ArgList.cpp

diff --git a/llvm/lib/Option/ArgList.cpp b/llvm/lib/Option/ArgList.cpp
index 2f4e212..9f9d63f 100644
--- a/llvm/lib/Option/ArgList.cpp
+++ b/llvm/lib/Option/ArgList.cpp
@@ -230,10 +230,8 @@
     HandleMultipleSubcommands(SubCommands);
     return {};
   }
-  if (!OtherPositionals.empty()) {
+  if (!OtherPositionals.empty())
     HandleOtherPositionals(OtherPositionals);
-    return {};
-  }
 
   if (SubCommands.size() == 1)
     return SubCommands.front();

llvm/unittests/Option/OptionSubCommandsTest.cpp

diff --git a/llvm/unittests/Option/OptionSubCommandsTest.cpp b/llvm/unittests/Option/OptionSubCommandsTest.cpp
index e31a326..d4744c9 100644
--- a/llvm/unittests/Option/OptionSubCommandsTest.cpp
+++ b/llvm/unittests/Option/OptionSubCommandsTest.cpp
@@ -192,6 +192,19 @@
         std::string::npos,
         ErrMsg.find("Option [lowercase] is not valid for SubCommand [bar]"));
   }
+
+  {
+    // Test case 7: Check valid use of a valid subcommand following more
+    // positional arguments.
+    const char *Args[] = {"bar", "input"};
+    InputArgList AL = T.ParseArgs(Args, MAI, MAC);
+    StringRef SC = AL.getSubCommand(
+        T.getSubCommands(), HandleMultipleSubcommands, HandleOtherPositionals);
+    EXPECT_EQ(SC, "bar"); // valid subcommand
+    EXPECT_NE(std::string::npos,
+              ErrMsg.find("Unregistered positionals passed"));
+    EXPECT_NE(std::string::npos, ErrMsg.find("input"));
+  }
 }
 
 TYPED_TEST(OptSubCommandTableTest, SubCommandHelp) {