[Flang] Generate documentation for compiler flags

This patch aims to create a webpage to document
Flang's command line options on https://flang.llvm.org/docs/
in a similar way to Clang's
https://clang.llvm.org/docs/ClangCommandLineReference.html

This is done by using clang_tablegen to generate an .rst
file from Options.td (which is current shared with Clang)
For this to work, ClangOptionDocEmitter.cpp was updated
to allow specific Flang flags to be included,
rather than bulk excluding clang flags.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D129864