Exposed a declarative way to specify that an attribute can be duplicated when merging attributes on a declaration. This replaces some hard-coded functionality from Sema.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199677 91177308-0d34-0410-b5e6-96231b3b80d8
Adding a bit of documentation that was missed with r198883 (when ParseArgumentsAsUnevaluated was added).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199679 91177308-0d34-0410-b5e6-96231b3b80d8