This patch fixes failure of the CodeGen/aix-ignore-xcoff-visibility.cpp test with command line round-trip.
The absence of '-fvisibility' implies '-mignore-xcoff-visibility'.
The problem is that when '-fvisibility default' is passed to -cc1, it isn't being generated. (This adheres to the principle that generation doesn't produce arguments with default values.)
However, that caused '-mignore-xcoff-visibility' to be implied in the generated command line (without '-fvisibility'), while it wasn't implied in the original command line (with '-fvisibility').
This patch fixes that by always generating '-fvisibility' and explains the situation in comment.
(The '-mginore-xcoff-visibility' option was added in D87451).