[clang][deps] Move enabling system header deps from `clang-scan-deps` to...

[clang][deps] Move enabling system header deps from `clang-scan-deps` to `DependencyScanning` library

This patch moves enabling system header deps from `clang-scan-deps` into the `DependencyScanning` library. This will make it easier to preserve semantics of the original TU command-line for modular dependencies (see D104036).

Reviewed By: arphaman

Differential Revision: https://reviews.llvm.org/D104033