[docs] Fix LibTooling documentation for OptionsParser change

[motemen]

Summary

Additional documentation fix per https://reviews.llvm.org/D94420

[serge-sans-paille]

I'm fine with the change. It would be great if this could literalinclude these two sample codes (as in https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#directive-literalinclude) and have them being compiled as part of the validation process. That way we would avoid such doc-bit-rotting (?) in the future. This require some effort so I'm still okay if you don't give it a try, in which case I will do. Please tell me!

[motemen]

Thank you for your review. Though I'm interested in it, I'm not familiar with the build system. So I won't go further in this patch.
Thanks!

clang/docs/LibTooling.rst

.. code-block:: c++
 
   using namespace clang::tooling;
 
   // Apply a custom category to all command-line options so that they are the
   // only ones displayed.
   static llvm::cl::OptionCategory MyToolCategory("my-tool options");
 
   int main(int argc, const char **argv) {
-    // CommonOptionsParser constructor will parse arguments and create a
-    // CompilationDatabase.  In case of error it will terminate the program.
-    CommonOptionsParser OptionsParser(argc, argv, MyToolCategory);
+    // CommonOptionsParser::create will parse arguments and create a
+    // CompilationDatabase.
+    auto ExpectedParser = CommonOptionsParser::create(argc, argv, MyToolCategory);
+    if (!ExpectedParser) {
+      // Fail gracefully for unsupported options.
+      llvm::errs() << ExpectedParser.takeError();
+      return 1;
+    }
+    CommonOptionsParser &OptionsParser = ExpectedParser.get();
 
     // Use OptionsParser.getCompilations() and OptionsParser.getSourcePathList()
     // to retrieve CompilationDatabase and the list of input file paths.
   }
 
 Creating and running a ClangTool
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
.. code-block:: c++
   // command-line options related to the compilation database and input files.
   // It's nice to have this help message in all tools.
   static cl::extrahelp CommonHelp(CommonOptionsParser::HelpMessage);
 
   // A help message for this specific tool can be added afterwards.
   static cl::extrahelp MoreHelp("\nMore help text...\n");
 
   int main(int argc, const char **argv) {
-    CommonOptionsParser OptionsParser(argc, argv, MyToolCategory);
+    auto ExpectedParser = CommonOptionsParser::create(argc, argv, MyToolCategory);
+    if (!ExpectedParser) {
+      // Fail gracefully for unsupported options.
+      llvm::errs() << ExpectedParser.takeError();
+      return 1;
+    }
+    CommonOptionsParser &OptionsParser = ExpectedParser.get();
     ClangTool Tool(OptionsParser.getCompilations(),
                    OptionsParser.getSourcePathList());
     return Tool.run(newFrontendActionFactory<clang::SyntaxOnlyAction>().get());
   }
 
 Running the tool on some code
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^