Add ByteAddressBuffer templated load/store tests

[kmpeng]

Closes #266.

Adds tests for ByteAddressBuffer's templated load/store member functions. These test one 32-bit type (uint), bool, one 16-bit type (int16_t), one 64-bit type (int64_t), a struct with an array in it, a small struct, and a big struct. I didn't test every single type of scalar since we're mostly trying to test the various different shapes that could be used.

[alsepkow]

Finn/Alex pair review: Left a few minor comments.

[kmpeng]

I have a question for people reviewing this: ByteAddressBuffer load/stores work with arrays in DXC (and explicitly tests them), but we agreed to move towards disallowing array returns in 202x. Should I keep the array test here? And should I implement array support for these methods in Clang?

[bogner]

I have a question for people reviewing this: ByteAddressBuffer load/stores work with arrays in DXC (and explicitly tests them), but we agreed to move towards disallowing array returns in 202x. Should I keep the array test here? And should I implement array support for these methods in Clang?

We discussed this elsewhere but for posterity I'll note it here: no, we shouldn't test the array behaviour or implement the array behaviour in clang if we're disallowing array returns in 202x. Tests that we reject array versions should be written, but those belong in the clang/dxc repos and don't need to be covered in the offload test suite here.

[alsepkow]

LGTM