[flang] Add -fpass-plugin option to Flang frontend

Add the -fpass-plugin option to flang which dynamically loads LLVM passes from the
shared object passed as the argument to the flag. The behavior of the option is
designed to replicate that of the same option in clang and thus has the same
capabilities and limitations.

- Multiple instances of -fpass-plugin=path-to-file can be specified and each of the
  files will be loaded in that order.

- The flag can be passed to both flang-new and flang-new -fc1.

Differential Revision: https://reviews.llvm.org/D129156