[DTLTO][PS4,PS5] Suppress system headers directory warning (#167685)
Our driver warns under various circumstances if SDK directories can't be
found.
This warning is not applicable in ThinLTO codegen mode
(-fthinlto-index=). Suppress it.
The motivation for doing this is that we sometimes see this warning
emitted when DTLTO invokes the compiler on a remote machine to
do the LTO backend compilations (with -fthinlto-index=).
Internal Ref: TOOLCHAIN-20592