[flang] Make the `flang` wrapper script check the Bash version

The `flang` wrapper script has been written in a relatively modern
version of Bash and it fails on systems with older versions. This patch
makes the script check the version of Bash being used to run it and
generates an error when unsupported version is used.

This was discussed in more detail in:
  * https://github.com/flang-compiler/f18-llvm-project/issues/1344.
Many thanks to Anthony Cabrera for the suggestion and for identifying
the oldest version of Bash that we can use here.

Differential Revision: https://reviews.llvm.org/D116608