[SPIR-V] Add capability for non-uniform indexing for `StructuredBuffer` types. (#163424)
- Capability `StorageBufferArrayNonUniformIndexing` is required if the
non-uniform index accesses "arrays in the StorageBuffer [storage
class](https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#Storage_Class)
or
BufferBlock-[decorated](https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#Decoration)
arrays." (e.g. `RWStructuredBuffer`, `StructuredBuffer`)
- Also fix the wrong unit test name: `StructuredBufferNonUniformIdx.ll`
-> `RWBufferNonUniformIdx.ll`
Resolves https://github.com/llvm/llvm-project/issues/162889
Addresses https://github.com/llvm/llvm-project/issues/161852