[Attributor] Deduce "nofree" function attribute

Summary: Deduce "nofree" function attribute. A more concise description of "nofree" is on D49165.

Reviewers: jdoerfert

Reviewed By: jdoerfert

Subscribers: homerdin, hfinkel, lebedev.ri, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62687

llvm-svn: 365924