[clang] [flang] [flang][driver] Support Makefile dependency generation (PR #209379)

[Tarun Prabhu via cfe-commits]

tarunprabhu wrote:

> Just making sure I'm reading things correctly, this is NFC as far as Clang is concerned because it really just moves code around, right? (I didn't see any functional changes aside from Options.td and Flang.cpp, but I wanted to make sure I wasn't missing something.)

Yes, that's right.

https://github.com/llvm/llvm-project/pull/209379