How to set header directory search path for virtually mapped files using `runToolOnCodeWithArgs`?

I was using runToolOnCodeWithArgs for unittesting.
There are some virtual headers in my test case.
I pass these headers with their file names to the argument VirtualMappedFiles of runToolOnCodeWithArgs.
But clang complains that these headers do not exist.

I have searched through all the testing code of clang but find nobody is using the VirtualMappedFiles argument.
Can you help me?
Or maybe it is suggested to use lit for my case?

Thank you.

Well, you need to specify the -I paths as necessary… Can you give a full command line you’re trying and perhaps a code snippet how you’re specifying the headers?