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.

tests/tests2/86-struct-init.expect

@@ -13,6 +13,10 @@ gv3: 7 8 9 a 68 6f 68 6f 0 0 0 0 0 0 0 0 0 0 0 0 31 32
 sinit16: 1 0 0 0 2 0 0 0
 gw: 1 2 3 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
 gsu: 5 6
+guv: 6 5 0 0
+guv.b: 5
+guv2: 8 7 0 0
+guv3: 7 8 0 0
 ls: 1 2 3 4
 ls2: 1 2 3 4
 lt: 68 65 6c 6c 6f 0 0 0 0 0 0 0 0 0 0 0 2a