Add a macro-based enumeration of all of the Decl nodes (like we do

Add a macro-based enumeration of all of the Decl nodes (like we do
with Stmt/Expr nodes), and convert some of the more mundane
switch-on-all-decl-kinds uses over to use this new file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@63570 91177308-0d34-0410-b5e6-96231b3b80d8