Refactored semantic handlers that would unilaterally apply the attribute to a...

Refactored semantic handlers that would unilaterally apply the attribute to a Decl so they all go through a common interface. No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195848 91177308-0d34-0410-b5e6-96231b3b80d8