Make attribute-target on a Definition-after-use update the LLVM attributes

Make attribute-target on a Definition-after-use update the LLVM attributes

As reported here: https://bugs.llvm.org/show_bug.cgi?id=36301
The issue is that the 'use' causes the plain declaration to emit
the attributes to LLVM-IR. However, if the definition added it
later, these would silently disappear.

This commit extracts that logic to its own function in CodeGenModule,
and has the attribute-applications done during 'definition' update
the attributes properly.

Differential Revision: https://reviews.llvm.org/D43095

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@324907 91177308-0d34-0410-b5e6-96231b3b80d8