[ASTScope] Eliminate getClosureIfClosureScope

... it's just a `dyn_cast` now.

Author: DougGregor

include/swift/AST/ASTScope.h

@@ -233,7 +233,6 @@ class ASTScopeImpl : public ASTAllocated<ASTScopeImpl> {
   /// Determine the kind of scope node we have.
   ScopeKind getKind() const { return kind; }
 
-  virtual NullablePtr<AbstractClosureExpr> getClosureIfClosureScope() const;
   ASTContext &getASTContext() const;
   NullablePtr<Decl> getDeclIfAny() const;
   NullablePtr<Stmt> getStmtIfAny() const;
@@ -1161,9 +1160,6 @@ class ClosureParametersScope final : public ASTScopeImpl {
   SourceRange
   getSourceRangeOfThisASTNode(bool omitAssertions = false) const override;
 
-  NullablePtr<AbstractClosureExpr> getClosureIfClosureScope() const override {
-    return closureExpr;
-  }
   Expr *getExpr() const { return closureExpr; }
   bool ignoreInDebugInfo() const override { return true; }
 

lib/AST/ASTScope.cpp

@@ -98,10 +98,11 @@ Pattern *AbstractPatternEntryScope::getPattern() const {
 }
 
 NullablePtr<AbstractClosureExpr> BraceStmtScope::parentClosureIfAny() const {
-  return !getParent() ? nullptr : getParent().get()->getClosureIfClosureScope();
-}
+  if (auto parent = getParent()) {
+    if (auto closureScope = dyn_cast<ClosureParametersScope>(parent.get()))
+      return closureScope->closureExpr;
+  }
 
-NullablePtr<AbstractClosureExpr> ASTScopeImpl::getClosureIfClosureScope() const {
   return nullptr;
 }