[llvm-reduce] Reducing attributes

Summary:
This handles all three places where attributes could currently be - `GlobalVariable`, `Function` and `CallBase`.
For last two, it correctly handles all three possible attribute locations (return value, arguments and function itself)

There was a previous attempt at it D73853,
which was committed in rGfc62b36a000681c01e993242b583c5ec4ab48a3c,
but then reverted all the way back in rGb12176d2aafa0ccb2585aa218fc3b454ba84f2a9
due to some (osx?) test failures.

Reviewers: nickdesaulniers, dblaikie, diegotf, george.burgess.iv, jdoerfert, Tyker, arsenm

Reviewed By: nickdesaulniers

Subscribers: wdng, MaskRay, arsenm, llvm-commits, mgorny

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83351