[flang] Update the regression tests to use the new driver when enabled

This patch updates most of the remaining regression tests (~400) to use
`flang-new` rather then `f18` when `FLANG_BUILD_NEW_DRIVER` is set.
This allows us to share more Flang regression tests between `f18` and
`flang-new`. A handful of tests have not been ported yet - these are
currently either failing or not supported by the new driver.

Summary of changes:
  * RUN lines in tests are updated to use `%flang_fc1` instead of `%f18`
  * option spellings in tests are updated to forms accepted by both `f18` and
    `flang-new`
  * variables in Bash scripts are renamed (e.g. F18 --> FLANG_FC1)
The updated tests will now be run with the new driver, `flang-new`,
whenever it is enabled (i.e when `FLANG_BUILD_NEW_DRIVER` is set).

Although this patch touches many files, vast majority of the changes are
automatic:
```
grep -IEZlr "%f18" flang/test/ | xargs -0 -l sed -i 's/%f18/%flang_fc1/g
```

Differential Revision: https://reviews.llvm.org/D100309