[ASTDumper] Print decl-refs, not full decls, for function captures.

This avoids infinite recursion when a function captures itself; e.g.,

```swift
func foo() {
  func bar() {
    bar()
  }
  bar()
}
```

It also just avoids printing unnecessarily verbose information,
since those decls would have already been dumped elsewhere in the
tree.

Author: allevato

lib/AST/ASTDumper.cpp

@@ -1484,7 +1484,11 @@ namespace {
         printFlag(value.isLocalCapture(), "is_local_capture");
         printFlag(value.isDynamicSelfMetadata(), "is_dynamic_self_metadata");
         if (auto *D = value.getDecl()) {
-          printRec(D, Label::always("decl"));
+          // We print the decl ref, not the full decl, to avoid infinite
+          // recursion when a function captures itself (and also because
+          // those decls are already printed elsewhere, so we don't need to
+          // print what could be a very large amount of information twice).
+          printDeclRefField(D, Label::always("decl"));
         }
         if (auto *E = value.getExpr()) {
           printRec(E, Label::always("expr"));

test/Frontend/ast-dump-json-no-crash.swift

@@ -491,3 +491,15 @@ func toReplaceWith(arg: Int) {}
 func moduleTypeUSRRegressionTest() {
     Swift.print("")
 }
+
+// Regression test: When a function captures another function, don't print the
+// entire captured decl. This causes infinite recursion in the dumper when a
+// local (nested) function captures itself.
+func outerFn() {
+    func innerFun(shouldRecurse: Bool) {
+        if shouldRecurse {
+            innerFun(shouldRecurse: false)
+        }
+    }
+    innerFun(shouldRecurse: true)
+}