Mark C++ reference parameters as dereferenceable

Because references must be initialized using some evaluated expression, they
must point to something, and a callee can assume the reference parameter is
dereferenceable. Taking advantage of a new attribute just added to LLVM, mark
them as such.

Because dereferenceability in addrspace(0) implies nonnull in the backend, we
don't need both attributes. However, we need to know the size of the object to
use the dereferenceable attribute, so for incomplete types we still emit only
nonnull.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@213386 91177308-0d34-0410-b5e6-96231b3b80d8
For i386, gcc 4.9 under Debian (at least) updated its paths to i586-linux-gnu
(i486-linux-gnu previously). Adding this triple to the list of search.
Also impacts clang 3.4.2

Reported on the Debian bug tracking system here:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=754963
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=755183




git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@213388 91177308-0d34-0410-b5e6-96231b3b80d8