[AST] Add a pointer to parent statement in 'CaseStmt'

[rintaro]

So that clients can easily get SwitchStmt or DoCatchStmt from a CaseStmt.

This is a preparation for on-demand VarDecl type checking in CaseStmt patterns. Similar to patterns in if/guard conditions, we want to type check VarDecl in switch/do-catch statement on demand.

[rintaro]

@swift-ci Please smoke test

[DougGregor]

Hrm. I support this is okay, but I wonder---could we instead ask ASTScope to provide this information for us, rather than add parent pointers each place we need to do this?

[rintaro]

Hm, interesting. Unfortunately, ASTScope is still disabled in code completion so far. I'll see if we can enable it soon.

[rintaro]

FWIW, only another place we want to add a parent pointer is a reference to TapExpr in VarDecl. Since VarDecl already have a pointer union to the parent PatternBindingDecl, VarDecl or Stmt, we can easily add Expr * to the union.

[DougGregor]

Okay, this is fine. I'd forgotten that ASTScope was not enabled in code completion. If we fix that, we can delete a nice pile of code.