[clang] [Clang][Driver] Final phase also determined by input (PR #218802)

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bgra8 wrote:

@Meinersbur we (inside google) also see huge compilation time regressions (5x slower compilation times in some cases).

This change introduced quadratic O(N²) time complexity into Clang's driver action construction.

Let's revert and re-submit with @nico 's proposal as fix.

https://github.com/llvm/llvm-project/pull/218802