To support the `-fvisibility=...` option in Flang, we need a pass to
rewrite all the global definitions in the LLVM dialect that have the
default visibility to have the specified visibility. This change adds
such a pass.
Note that I did not add an option for `visiblity=default`; I believe
this makes sense for compiler drivers since users may want to tack an
option on at the end of a compile line to override earlier options, but
I don't think it makes sense for this pass to accept
`visibility=default`--it would just be an early exit IIUC.
568ce76c6e