Add regression test for designated initializers through anonymous unions

[lm-sousa]

Clava should add a regression test for designated initializers that select fields through anonymous unions and structs.

This is the exact input used for the corresponding clang-dumper regression test:

struct anonymous_union {
  union {
    int value;
  };
};

struct nested_anonymous_union {
  union {
    struct {
      int value;
    };
  };
};

struct anonymous_union first = { .value = 1 };
struct nested_anonymous_union second = { .value = 2 };

Parse this input with the clang-dumper data and regenerate the code through Clava. The anonymous union and struct declarations should remain anonymous, and the initializers should be reconstructed with the source spelling:

struct anonymous_union first = { .value = 1 };
struct nested_anonymous_union second = { .value = 2 };

In particular, Clava must emit .value, not ..value, for both the direct and nested cases. Formatting may differ, but the designated initializer structure must remain faithful.

The clang-dumper fix is tracked in specs-feup/clang-dumper#48 and PR #49. The producer fix removes synthetic unnamed path entries from the source-level designator list. This issue covers adding the Clava consumer test and confirming that the available data is sufficient for faithful reconstruction.