[Tooling] Expose ExecutorConcurrency option.

[DiegoAstiazaran]

Summary

D65628 requires a flag to specify the number of threads for a clang-doc step. It would be good to use ExecutorConcurrency after exposing it instead of creating a new one that has the same purpose.

[sammccall]

Only hesitation is that AllTUsExecution is one executor of (potentially) many, so this is a weird place for a common flag.

However it's already "global" in the sense that --help will show it, so we may as well reuse it.
Maybe it should be moved to another file at some point.

cfe/trunk/include/clang/Tooling/AllTUsExecution.h

private:
   llvm::Optional<CommonOptionsParser> OptionsParser;
   const CompilationDatabase &Compilations;
   std::unique_ptr<ToolResults> Results;
   ExecutionContext Context;
   llvm::StringMap<std::string> OverlayFiles;
   unsigned ThreadCount;
 };
 
+extern llvm::cl::opt<unsigned> ExecutorConcurrency;
 extern llvm::cl::opt<std::string> Filter;
 
 } // end namespace tooling
 } // end namespace clang
 
 #endif // LLVM_CLANG_TOOLING_ALLTUSEXECUTION_H

cfe/trunk/lib/Tooling/AllTUsExecution.cpp

llvm::Error AllTUsToolExecutor::execute(
   }
 
   if (!ErrorMsg.empty())
     return make_string_error(ErrorMsg);
 
   return llvm::Error::success();
 }
 
-static llvm::cl::opt<unsigned> ExecutorConcurrency(
+llvm::cl::opt<unsigned> ExecutorConcurrency(
     "execute-concurrency",
     llvm::cl::desc("The number of threads used to process all files in "
                    "parallel. Set to 0 for hardware concurrency. "
                    "This flag only applies to all-TUs."),
     llvm::cl::init(0));
 
 class AllTUsToolExecutorPlugin : public ToolExecutorPlugin {
 public: