Switch Decl instantiation to DeclNodes.inc.

This replaces a long list of declarations for visitor functions with
a list generated from DeclNodes.inc.  Nothing really interesting came
out of it; we had comprehensive coverage anyway
(excluding FriendTemplateDecls).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185118 91177308-0d34-0410-b5e6-96231b3b80d8