[flang][hlfir] Separate -emit-fir and -emit-hlfir for bbc

In review for https://reviews.llvm.org/D146278, @vzakhari asked to
separate -emit-fir and -emit-hlfir. This will allow FIR to be easily
outputted after the HLFIR passes have been run.

The new semantics are as follows:

| Action      | -hlfir? | Result                          |
| =========== | ======= | =============================== |
| -emit-hlfir | N       | Outputs HLFIR                   |
| -emit-hlfir | Y       | Outputs HLFIR                   |
| -emit-fir   | N       | Outputs FIR, using old lowering |
| -emit-fir   | Y       | Outputs FIR, lowering via HLFIR |

This is tested in flang/test/HLFIR/hlfir-flags.f90

Depends on: D151088

Differential Revision: https://reviews.llvm.org/D151108