Merge pull request #14945 from jrose-apple/frozen-enums

Implementation for /most/ of SE-0192 (frozen and non-frozen enums)

include/swift/AST/Decl.h

@@ -3224,6 +3224,13 @@ public:
   bool isIndirect() const {
     return getAttrs().hasAttribute<IndirectAttr>();
   }
+
+  /// True if the enum can be exhaustively switched within \p useDC.
+  ///
+  /// Note that this property is \e not necessarily true for all children of
+  /// \p useDC. In particular, an inlinable function does not get to switch
+  /// exhaustively over a non-exhaustive enum declared in the same module.
+  bool isExhaustive(const DeclContext *useDC) const;
 };
 
 /// StructDecl - This is the declaration of a struct, for example: