[InlineAdvisor] Allow loading advisors as plugins

Adds the ability to load InlineAdvisors as plugins. This allows developing and distributing inlining heuristics outside of tree.

The PluginInlineAdvisorAnalysis class serves as the entry point for dynamic advisors. Plugins must register instances of this class to provide their own InliningAdvisor.

Reviewed By: mtrofin

Differential Revision: https://reviews.llvm.org/D139644