How to add custom include dir to the header search path?

I'm writing a tools (do some static check) using clang lib . This is my code:

void main()
{
    DiagnosticOptions diagnosticOptions;
    TextDiagnosticPrinter *printer = new
TextDiagnosticPrinter(llvm::nulls(),
        diagnosticOptions);
    llvm::IntrusiveRefCntPtr<clang::DiagnosticIDs> diagIDs;
    DiagnosticsEngine diagnostics(diagIDs, printer);
    LangOptions langOpts;

    TargetOptions targetOpts;
    targetOpts.Triple = llvm::sys::getDefaultTargetTriple();
    TargetInfo *pti = TargetInfo::CreateTargetInfo(diagnostics, targetOpts);
    FileSystemOptions fsopts;
    FileManager fileManager(fsopts);
    SourceManager sourceManager(diagnostics, fileManager);

    HeaderSearch headerSearch(fileManager, diagnostics, langOpts, pti);

    CompilerInstance *theCompInst = new CompilerInstance();
    theCompInst->setDiagnostics(&diagnostics);
    Preprocessor *pp = new Preprocessor(diagnostics, langOpts,
pti,sourceManager, headerSearch, *theCompInst);

    const FileEntry *pFile = fileManager.getFile(file_name);
    sourceManager.createMainFileID(pFile);
   
theCompInst->getDiagnosticClient().BeginSourceFile(theCompInst->getLangOpts(),pp);
    theCompInst->setPreprocessor(pp);
    theCompInst->setSourceManager(&sourceManager);
    theCompInst->setTarget(pti);
    theCompInst->setDiagnostics(&diagnostics);
    SourceManager &sourceMgr = theCompInst->getSourceManager();
    theCompInst->createASTContext();
    Rewriter rewriter;
    rewriter.setSourceMgr(sourceMgr,langOpts);

    MyASTConsumer consumer(rewriter);
    ParseAST(theCompInst->getPreprocessor(), &consumer,
theCompInst->getASTContext());
    theCompInst->getDiagnosticClient().EndSourceFile();
}

I want to check some C/C++ code using this tools. But *.h file and *.c file
is not in the same filefold.

Example:
  test.c context:
    #include "test.h"
     .........

test.c in D:\test\test.c
test.h in D:\test\include\test.h

So I need to add "D:\test\include" to the header search path. How to do that
in my code?
Thanks all!