Missing support for anonymous fields

[ttreyer]

A structure can have multiple nested anonymous fields, as demonstrated by struct Foo2 in "tests/data/data_source.c":

struct Foo2 {
  int a;
  union {
    struct Foo1 f;
    struct {
      char g;
    };
  };
};

However, bpftrace reports only two fields being in struct Foo2 and there is no way for the user to access the content of the anonymous records.
We should enable the following syntax for the user to access these fields: foo2->g.

What reproduces the bug? Provide code if possible.

Remove the GTEST_SKIP() at the top of the test "field_analyser_dwarf.parse_struct_anonymous_fields".