[Driver] Allow users to silence the warning that is issued when the
deployment target is earlier than iOS 11 and the target is 32-bit.

This is a follow-up to r306922.

rdar://problem/32230613

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309607 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 4633b85c2c9915c7b57fb2327912a0cc59e1b077)

Conflicts:
lib/Driver/ToolChains/Darwin.cpp
4 files changed