ParseHelper: don't assign xfb_offset for struct members

This commit undoes the code on the commit "ParseHelper: assign
xfb_offset for struct members too" (commit
af8c1bdb), except the method renaming,
and updates the tests.

During the review of the implementation of Vulkan extension
VK_EXT_transform_feedback for the Linux Intel driver, it was pointed
that the Vulkan environment spec for SPIR-V only ever refers to block
or block members being decorated. It would be strange to not do the
same for OpenGL (ARB_gl_spirv). That would also fit better to what
GLSL does, where setting explicit xfb offsets for struct members is
not allowed.

FWIW, the original patch was proposed based on the fact that
ARB_gl_spirv is relying on OpenGL for how xfb offsets should be
assigned to members, and it was not clear (at least to me) which is
the responsible of such.