Make MandatoryInlining a SILFunctionTransform

[zoecarver]

This patch is the first in a few that will clean up MandatoryInlining. For ease of reviewing, I am only changing MandatoryInlining from a SILModuleTransform to a SILFunctionTransform in this patch. Next, I'm going to remove the function removal cleanup from MandatoryInlining and move any missing functionality to MandatoryCombine.

cc @gottesmm

[gottesmm]

A few things:

1. Part of this is that the design will need to be changed. Mandatory Inlining currently processes functions recursively and processes Callees. Instead what we should do is rely on the pass manager to instead schedule those changes.

2. It is important that you make sure that when mandatory inlining links in functions, the linked in function is added to the pass manager worklist.

[gottesmm]

I would also look at how the performance inlined handles this:

https://github.com/apple/swift/blob/master/lib/SILOptimizer/Transforms/PerformanceInliner.cpp

[zoecarver]

I realized that I couldn't make this a SILFunctionTransform without trying to remove every function in the module. #28396 fixes that. I think its best if I only move forward with #28396.

@gottesmm thank you for the comments; I'll address them in my other patch.