[LangRef] Explicitly allow readnone and reaodnly functions to unwind

Summary:
This change edits the language reference to explicitly allow the
existence of readnone and readonly functions that can throw.  Full
discussion at
http://lists.llvm.org/pipermail/llvm-dev/2017-January/108637.html

Reviewers: dberlin, chandlerc, hfinkel, majnemer

Reviewed By: majnemer

Subscribers: majnemer, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D28740

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295000 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed