[OpenCL] Add 'denorms-are-zero' function attribute

Summary:
Generate attribute 'denorms-are-zero'='true' if '-cl-denorms-are-zero'
compile option was specified and 'denorms-are-zero'='false' otherwise.

Patch by krisb

Reviewers: Anastasia, yaxunl

Reviewed By:  yaxunl 

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D45808

llvm-svn: 330404