Implement scoped enumerations

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We want enumerators to be scoped to their enclosing enumerations. That is:

def X : typename = enum {
  A, B, C
};

def x1 : X = X::A; // OK
def x2 : X = A;    // error

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Implemented in 1dbd06e22fb167c907b08d02383d3c105b920025.