[NFC] Removing experimental designation for ninja in docs.

[ncv]

Summary

Ninja is no longer an experimental tool, documentation changed to
reflect this.

clang/docs/HowToSetupToolingForLLVM.rst

clang::ASTConsumer *newASTConsumer() {
       if (this->ASTDump.operator _Bool())
           return clang::CreateASTDumper(nullptr /*Dump to stdout.*/,
                                         this->ASTDumpFilter);
       if (this->ASTPrint.operator _Bool())
           return clang::CreateASTPrinter(&llvm::outs(), this->ASTDumpFilter);
       return new clang::ASTConsumer();
   }
 
-(Experimental) Using Ninja Build System
+Using Ninja Build System
 =======================================
 
 Optionally you can use the `Ninja <https://github.com/martine/ninja>`_
 build system instead of make. It is aimed at making your builds faster.
 Currently this step will require building Ninja from sources.
 
 To take advantage of using Clang Tools along with Ninja build you need
 at least CMake 2.8.9.

 
 Now you are ready to build and test LLVM using Ninja:
 
 .. code-block:: console
 
   $ ninja check-all
 
 Other target names can be used in the same way as with make.
-