Removing the notion of TargetAttributesSema and replacing it with one where the parse...

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