Add the .Self name for the type expression of a compile time binding (#5937)

We add a virtual node (`CompileTimeBindingPatternStart`) as the first
child of `CompileTimeBindingPattern` which holds the identifier
underneath it, so that it is checked just before the type expression of
the `CompileTimeBindingPattern`. When we reach this virtual node during
check, we add `.Self` as a name in the current scope, and when we reach
`CompileTimeBindingPattern` we remove it from scope, which ensures it's
present during only the checking of the type expression for the compile
time pattern.

At the moment the `.Self` has a different type (it's a `TypeType`) than
other `.Self` in the facet type (which are a single `FacetType`), but
the intention is to immediately substitute it out of the facet type
entirely, replacing it with a reference to the compile time binding (a
`BindSymbolicName`) itself. A TODO has been added for this.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/node_stack.h

@@ -479,6 +479,7 @@ class NodeStack {
       case Parse::NodeKind::CallExprComma:
       case Parse::NodeKind::ChoiceAlternativeListComma:
       case Parse::NodeKind::CodeBlock:
+      case Parse::NodeKind::CompileTimeBindingPatternStart:
       case Parse::NodeKind::ContinueStatementStart:
       case Parse::NodeKind::CorePackageName:
       case Parse::NodeKind::ExportIntroducer: