Refactor Clang sema attribute handling.

This submission improves Clang sema handling by using Clang tablegen
to generate common boilerplate code. As a start, it implements AttributeList
enumerator generation and case statements for AttributeList::getKind.

A new field "SemaHandler" is introduced in Attr.td and by default set to 1
as most of attributes in Attr.td have semantic checking in Sema. For a small
number of attributes that don't appear in Sema, the value is set to 0.

Also there are a small number of attributes that only appear in Sema but not
in Attr.td. Currently these attributes are still hardcoded in Sema AttributeList.

Reviewed by Delesley Hutchins.

llvm-svn: 152169