[clang][deps] NFC: Preserve the original frontend action

This patch stops adjusting the frontend action when `clang-scan-deps` is configured to use the full output format.

In a future patch, the dependency scanner needs to check whether the original compiler invocation builds a PCH. That's impossible when `-Eonly` et al. override `-emit-pch`.

The `-Eonly` flag is not needed - the dependency scanner explicitly sets up its own frontend action anyways.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D103461