[LTO][clang] Teaching Clang to Pass Plugin Options to the AIX Linker

This patch teaches `clang` to use the prefix `-bplugin_opt:` (instead of `-plugin-opt`) on AIX, when passing plugin options to the linker. This patch follows https://reviews.llvm.org/D134668.

We put the code that decides what plugin option prefix to use at the top of the function `tools::addLTOOptions`. The plugin option prefix, the mcpu prefix, and the opt level prefix are different on AIX. We thought about choosing the strings in a function that reads the linker name and the target triple, or we could push the logic into different derived `ToolChain` classes. But this logic would not be used anywhere else, so these alternatives looked too complicated for what they did. Therefore we are doing it the current way. That said, I am all ears for suggestions to improve this code!

Subsequent code uses the `PluginOptPrefix` variable consistently instead of the hardcoded `-plugin-opt`.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D134820