Pull the Attr iteration parts out of Attr.h, so including DeclBase.h doesn't pull in all the generated Attr code.

Required to pull some functions out of line, but this shouldn't have a perf impact.
No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@169092 91177308-0d34-0410-b5e6-96231b3b80d8