struct-init: Correctly parse unnamed member initializers

struct-init: Correctly parse unnamed member initializers

For
  union U { struct {int a,b}; int c; };
  union U u = {{ 1, 2, }};
The unnamed first member of union U needs to actually exist in the
structure so initializer parsing isn't confused about the double braces.
That means also the a and b members must be part of _that_, not of
union U directly.  Which in turn means we need to do a bit more work
for field lookup.

See the testcase extension for more things that need to work.