[swift-api-digester] Ensure we exclude decls of low accessibility from the API comparison. (#5382)

test/api-digester/Inputs/cake.swift

@@ -1,4 +1,11 @@
 public struct S1 {
   public func foo1() {}
   mutating public func foo2() {}
+  internal func foo3() {}
+  private func foo4() {}
+  fileprivate func foo5() {}
+}
+
+public class C1 {
+	open func foo1() {}
 }

test/api-digester/Outputs/cake.json

@@ -62,6 +62,49 @@
           ]
         }
       ]
+    },
+    {
+      "kind": "TypeDecl",
+      "name": "C1",
+      "printedName": "C1",
+      "declKind": "Class",
+      "usr": "s:C4cake2C1",
+      "location": "",
+      "moduleName": "cake",
+      "children": [
+        {
+          "kind": "Function",
+          "name": "foo1",
+          "printedName": "foo1()",
+          "declKind": "Func",
+          "usr": "s:FC4cake2C14foo1FT_T_",
+          "location": "",
+          "moduleName": "cake",
+          "children": [
+            {
+              "kind": "TypeNominal",
+              "name": "Void",
+              "printedName": "()"
+            }
+          ]
+        },
+        {
+          "kind": "Constructor",
+          "name": "init",
+          "printedName": "init()",
+          "declKind": "Constructor",
+          "usr": "s:FC4cake2C1cFT_S0_",
+          "location": "",
+          "moduleName": "cake",
+          "children": [
+            {
+              "kind": "TypeNominal",
+              "name": "C1",
+              "printedName": "C1"
+            }
+          ]
+        }
+      ]
     }
   ]
 }

tools/swift-api-digester/swift-api-digester.cpp

@@ -1112,6 +1112,15 @@ static bool shouldIgnore(Decl *D) {
       return true;
     if (VD->getName().empty())
       return true;
+    switch (VD->getFormalAccess()) {
+    case Accessibility::Internal:
+    case Accessibility::Private:
+    case Accessibility::FilePrivate:
+      return true;
+    case Accessibility::Public:
+    case Accessibility::Open:
+      break;
+    }
   }
 
   if (auto *ClangD = D->getClangDecl()) {