Correct field lookup in concept types

Fixes #6770

tests/concepts/t6770.nim

@@ -0,0 +1,27 @@
+discard """
+output: '''
+10
+10
+'''
+"""
+
+type GA = concept c
+  c.a is int
+
+type A = object
+  a: int
+
+type AA = object
+  case exists: bool
+  of true:
+    a: int
+  else:
+    discard
+
+proc print(inp: GA) =
+  echo inp.a
+
+let failing = AA(exists: true, a: 10)
+let working = A(a:10)
+print(working)
+print(failing)