Turn delicate case of designated re-initialization into error (#70)

Consider:
  struct P { int x, y; }
  struct S { struct P p; }

  struct P p0 = { 1,2 };
  struct S s1 = { .p = p0; .p.x = 3 };

ISO C99 and recent versions of Clang initialize s1.p.y to 2, i.e.
the initialization of s1.p.y to p0.y implied by ".p = p0" is kept,
even though the initialization of s1.p.x to p0.x is overwritten
by ".p.x = 3".

GCC, old versions of Clang, and previous versions of CompCert
initialize s1.p.y to the default value 0.  I.e. the initialization
".p = p0" is forgotten, leaving default values for the fields of .p
before ".p.x = 3" takes effect.

Implementing the proper ISO C99 semantics in CompCert is difficult,
owing to a mismatch between the intended semantics and the C.init
representation of initializers.

This commit turns the delicate case of reinitialization above
(re-initializing a member of a composite that has already been
initialized as a whole) into a compile-time error.

We will then see if the delicate case occurs in practice and needs
further attention.

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