[SPIRV][NFCI] Use unordered data structures for SPIR-V extensions (#183567)
Review follow-up from https://github.com/llvm/llvm-project/pull/183325
No reason for these data structures to be ordered.
Minor annoyance when trying to use `DenseMap` because of the C++ code
for enums generated by TableGen, but not too bad.
Signed-off-by: Nick Sarnie <nick.sarnie@intel.com>