swift234

AST: Add dynamic cast from Decl to IterableDeclContext

Also allow cast<> from any Decl to DeclContext, not just ValueDecl.
An example of a Decl that is a DeclContext but not a ValueDecl is
ExtensionDecl.

The const_cast<> hack is horrid but its needed to deal with both const
and non-const operands in the patch that eventually uses this.

Note that this adds an explicit 'const auto' to the prior usage of
cast<DeclContext> in Mangling.cpp.

NFC

Progress on <rdar://problem/20981254>.

Swift SVN r28929