[clang][cli] NFC: Remove intermediate command line parsing functions

Patch D96280 moved command line round-tripping from each parsing functions into single `CreateFromArgs` function.

This patch cleans up the individual parsing functions, essentially merging `ParseXxxImpl` with `ParseXxx`, as the distinction is not necessary anymore.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D96323