[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