add testing for SPIR-V 1.5

[bashbaug]

fixes #2140

Adds testing for SPIR-V 1.5 features:

• Adds a test for bitcasts between pointers and vectors of integers. Note, SPIR-V 1.5 only supports bitcasts to vectors of two 32-bit integers. Therefore, the SPIR-V 1.5 behavior will only be exercised on devices with 64-bit pointers. The test will run on devices with 32-bit pointers, but will instead bitcast to scalars.
• Adds a test for OpGroupNonUniformBroadcast with a dynamic index. Note, this is not an exhaustive test, and only unsigned integer types are tested, to avoid duplicating testing for cl_khr_subgroup_ballot.

[bashbaug]

Note: Implementations using the SPIR-V LLVM Translator will likely need this fix to pass the bitcast test: KhronosGroup/SPIRV-LLVM-Translator#2948

[svenvh]

Looks good to me, just a few nitpicks.