[flang][driver] Add support for `-debug-dump-pft`

This patch adds support for dumping the pre-FIR tree in `flang-new
-fc1`, i.e. Flang's frontend driver. This flag is functionally identical
to `-pft-test` in `bbc` and semantically similar to
`-fdebug-dump-parse-tree` from `flang-new -fc1`.

Differential Revision: https://reviews.llvm.org/D121198