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

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      | -flang-experimental-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 |

A patch for bbc will follow.

Differential Revision: https://reviews.llvm.org/D151088