API checker: only diagnose adding enum cases to exhaustive enums

Adding new cases to a non-exhaustive enum type isn't source-breaking
since it only triggers a warning.

rdar://63464929

tools/swift-api-digester/ModuleAnalyzerNodes.h

@@ -524,6 +524,7 @@ class SDKNodeDeclType: public SDKNodeDecl {
   // Check whether the type declaration is pulled from an external module so we
   // can incorporate extensions in the interested module.
   bool IsExternal;
+  bool IsEnumExhaustive;
   bool HasMissingDesignatedInitializers;
   bool InheritsConvenienceInitializers;
 public:
@@ -550,6 +551,11 @@ public:
     return EnumRawTypeName;
   }
 
+  bool isEnumExhaustive() const {
+    assert(isEnum());
+    return IsEnumExhaustive;
+  }
+
   bool hasMissingDesignatedInitializers() const {
     return HasMissingDesignatedInitializers;
   };