[bugpoint][PR29027] Reduce function attributes

Summary:
In addition to reducing the functions in an LLVM module, bugpoint now
reduces the function attributes associated with each of the remaining
functions.

To test this, add a -bugpoint-crashfuncattr test pass, which crashes if
a function in the module has a "bugpoint-crash" attribute. A test case
demonstrates that the IR is reduced to just that one attribute.

Reviewers: MatzeB, silvas, davide, reames

Reviewed By: reames

Subscribers: reames, llvm-commits

Differential Revision: https://reviews.llvm.org/D55216

llvm-svn: 349601