[spirv] Change enum case uniquing in gen_spirv_dialect.py

In SPIR-V we can have multiple symbols corresponding to the same
enum value. This is because when an extension is introduced into
the core spec, its suffix is typically removed, e.g., 'VulkanKHR'
memory model becomes 'Vulkan' memory model in SPIR-V 1.5.

Previously we just keep the first symbol for an enum value. That
symbol is not necessarily a better one. This CL changes to sort
symbols, grouped by enum values, alphabetically and then keep
the first one, which is typically shorter and without the extension
suffix. We also fix up certain ones like HlslSemanticGOOGLE.

PiperOrigin-RevId: 272290363