Avoid symbolic witnesses for .Self in an impl decl (#7564)

Point symbolic witnesses into `.Self` written inside an impl decl at the
impl that is being declared. This is tricky because the impl does not
yet exist. So we use a new instruction `ImplSelfWitness` which _will_ be
replaced by the `ImplWitness` once it becomes available. The
`ImplSelfWitness` acts like a symbolic witness, except it does not
perform lookup, since we know which impl we will get a witness from.

This prevents us from finding other impls when performing lookups into
`.Self` in an impl decl, which produces incorrect/incoherent results.

toolchain/check/node_stack.h

@@ -481,7 +481,6 @@ class NodeStack {
       case Parse::NodeKind::VariableIntroducer:
       case Parse::NodeKind::InlineImportBody:
       case Parse::NodeKind::InlineIntroducer:
-      case Parse::NodeKind::WhereOperand:
         return Id::Kind::None;
       case Parse::NodeKind::AdaptIntroducer:
       case Parse::NodeKind::AliasInitializer:
@@ -543,6 +542,7 @@ class NodeStack {
       case Parse::NodeKind::StructTypeLiteralComma:
       case Parse::NodeKind::TerseBodyArrow:
       case Parse::NodeKind::TupleLiteralComma:
+      case Parse::NodeKind::WhereOperand:
       case Parse::NodeKind::WhileCondition:
         return Id::Kind::Invalid;
       default: