[Inline] Copy "null-pointer-is-valid" attribute in caller.

Summary:
Normally, inling does not happen if caller does not have
"null-pointer-is-valid"="true" attibute but callee has it.

However, alwaysinline may force callee to be inlined.
In this case, if the caller has the "null-pointer-is-valid"="true"
attribute, copy the attribute to caller.

Reviewers: efriedma, a.elovikov, lebedev.ri, jyknight

Reviewed By: efriedma

Subscribers: eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D50000

llvm-svn: 338292