Suggest people use -Xclang not -cc1 when passing options to the frontend.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192222 91177308-0d34-0410-b5e6-96231b3b80d8

docs/IntroductionToTheClangAST.rst

@@ -46,9 +46,9 @@ Let's look at a simple example AST:
       return result;
     }
 
-    # Clang by default is a frontend for many tools; -cc1 tells it to directly
-    # use the C++ compiler mode.
-    $ clang -cc1 -ast-dump test.cc
+    # Clang by default is a frontend for many tools; -Xclang is used to pass
+    # options directly to the C++ frontend.
+    $ clang -Xclang -ast-dump -fsyntax-only test.cc
     TranslationUnitDecl 0x5aea0d0 <<invalid sloc>>
     ... cutting out internal declarations of clang ...
     `-FunctionDecl 0x5aeab50 <test.cc:1:1, line:4:1> f 'int (int)'