[InlineOrder] Plugin Inline Order

This allows developing and distributing inlining heuristics
outside of tree. And together with the inline advisor plugins
allows for fine grained control of the inliner.

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

I'm checking in this patch on behalf of ibricchi
<ibricchi@student.ethz.ch>.

Differential Revision: https://reviews.llvm.org/D140637