Fixing small errors in libAST Matcher Tutorial.

[madsravn]

Summary

The tutorial contains a few errors which results in code not being able to compile.

One error was described here: https://llvm.org/bugs/show_bug.cgi?id=25583 .

I found and fixed the error and one additional error.

[aaron.ballman]

LGTM, thank you for the documentation fix!

docs/LibASTMatchersTutorial.rst

.. code-block:: c++
                       hasRHS(expr(hasType(isInteger())))))).bind("forLoop");
 
 And change ``LoopPrinter::run`` to
 
 .. code-block:: c++
 
       void LoopPrinter::run(const MatchFinder::MatchResult &Result) {
         ASTContext *Context = Result.Context;
-        const ForStmt *FS = Result.Nodes.getStmtAs<ForStmt>("forLoop");
+        const ForStmt *FS = Result.Nodes.getNodeAs<ForStmt>("forLoop");
         // We do not want to convert header files!
-        if (!FS || !Context->getSourceManager().isFromMainFile(FS->getForLoc()))
+        if (!FS || !Context->getSourceManager().isWrittenInMainFile(FS->getForLoc()))
           return;
         const VarDecl *IncVar = Result.Nodes.getNodeAs<VarDecl>("incVarName");
         const VarDecl *CondVar = Result.Nodes.getNodeAs<VarDecl>("condVarName");
         const VarDecl *InitVar = Result.Nodes.getNodeAs<VarDecl>("initVarName");
 
         if (!areSameVariable(IncVar, CondVar) || !areSameVariable(IncVar, InitVar))
           return;
         llvm::outs() << "Potential array-based loop discovered.\n";