Add support for WGSL's atomicCompareExchangeWeak with the `__atomic…

[aweinstock314]

…_compare_exchange_result` struct, and add SPIR-V codegen for it.

Partially addresses gpuweb/gpuweb#2113, #1755.

[teoxoy]

Looks good, just missing some validation here:

naga/src/valid/expression.rs

Line 1395 in 2dcb696

comparison: _,

We should check that the comparison type is of the form:

struct __atomic_compare_exchange_result<T> {
  old_value : T; // old value stored in the atomic
  exchanged : bool; // true if the exchange was done
}

[teoxoy]

Good stuff!