[clang] [PassBuilder] Add a mechanism for adding passbuilder callbacks for static builds (PR #70171)

[William Moses via cfe-commits]

wsmoses wrote:

> acks presently can be done one of two ways:
> 
> * a shared library plugin
> * modifying the extensions list (and thus extensions.def)
> 
> This prevents the use of such functionality for static builds on build systems that cannot be modified, as well as prevents such functionality from being usable when using llvm/clang as a library.
> 
> This creates a third mechanism for adding these callbacks, an explicit list of said callbacks. It's otherwise identical to the plugin approach, except now instead of the list of plugins each being queried for the callback f

Will add later today

https://github.com/llvm/llvm-project/pull/70171