GLSL: XFB: more support for built-in block redeclarations with XFB.

- correct inheritence (or not) of the right XFB buffer
- compute implicit stride (fixes #1212)
- semantic check block-member redeclarations
- inherit stride from a member

gtests/Spv.FromFile.cpp

@@ -331,6 +331,8 @@ INSTANTIATE_TEST_CASE_P(
         "spv.precise.tese",
         "spv.precise.tesc",
         "spv.xfb.vert",
+        "spv.xfb2.vert",
+        "spv.xfb3.vert",
     })),
     FileNameAsCustomTestSuffix
 );