[flang] Rename `flang` as `flang-to-external-fc`

Our ultimate goal is to rename `flang-new` as `flang`. This is "Step 1"
in which we only rename the bash wrapper script, `flang`, as
`flang-to-external-fc`. The new name more accurately reflects what the
wrapper script does.

With this patch, `flang` becomes a symbolic link pointing to
`flang-to-external-fc`. This allows a smooth transmission for any
buildbots that use this script. Once the buildbots are reconfigured to
use the updated name, CMake rule for the symbolic link will be removed.

For more context see:
  * https://discourse.llvm.org/t/rename-flang-new-as-flang,
  * https://reviews.llvm.org/D125788.

Differential Revision: https://reviews.llvm.org/D125832