[Attributor] Deduce "no-capture" argument attribute

Add the no-capture argument attribute deduction to the Attributor
fixpoint framework.

The new string attributed "no-capture-maybe-returned" is introduced to
allow deduction of no-capture through functions that "capture" an
argument but only by "returning" it. It is only used by the Attributor
for testing.

Differential Revision: https://reviews.llvm.org/D59922

llvm-svn: 370817