[HeaderSearch] Use `isImport` only for imported headers and not for `#pragma once`.

There is a separate field `isPragmaOnce` and when `isImport` combines
both, it complicates HeaderFileInfo serialization as `#pragma once` is
the inherent property of the header while `isImport` reflects how other
headers use it. The usage of the header can be different in different
contexts, that's why `isImport` requires tracking separate from `#pragma once`.

Differential Revision: https://reviews.llvm.org/D104351