[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