Removing the notion of TargetAttributesSema and replacing it with one where the parsed attributes are responsible for knowing their target-specific nature, instead of letting Sema figure it out. This is necessary so that __has_attribute can eventually determine whether a parsed attribute applies to the given target or not.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198896 91177308-0d34-0410-b5e6-96231b3b80d8