ParseHelper: assign xfb_offset for struct members too

If the out variable is a struct type, with a xfb_offset explicitly
assigned, the members need to get their xfb_offset assigned. This is
specially relevant, as we cannot use layout qualifiers on struct
members.

gtests/Spv.FromFile.cpp

@@ -440,6 +440,7 @@ INSTANTIATE_TEST_CASE_P(
         "spv.rankShift.comp",
         "spv.specConst.vert",
         "spv.OVR_multiview.vert",
+        "spv.xfbOffsetOnStructMembersAssignment.vert",
     })),
     FileNameAsCustomTestSuffix
 );