[Attr] Add "willreturn" function attribute

This patch introduces a new function attribute, willreturn, to indicate
that a call of this function will either exhibit undefined behavior or
comes back and continues execution at a point in the existing call stack
that includes the current invocation.

This attribute guarantees that the function does not have any endless
loops, endless recursion, or terminating functions like abort or exit.

Patch by Hideto Ueno (@uenoku)

Reviewers: jdoerfert

Subscribers: mehdi_amini, hiraditya, steven_wu, dexonsmith, lebedev.ri, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62801

llvm-svn: 364555