Break the assumption that any sort of scope (e.g. a loop scope) can

Break the assumption that any sort of scope (e.g. a loop scope) can
hold declarations.  Instead, introduce a new "DeclScope" scope type that
holds them explicitly.  For now, all scopes have this bit, but in the
future we can use them to fix some issues Neil noticed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@41431 91177308-0d34-0410-b5e6-96231b3b80d8