[0004] Adding spec language for unions

[llvm-beanz]

This fleshes out the language design for union types.

Resolves #38, resolves #523

[llvm-beanz]

I think this PR also resolves #50, but would like @s-perron's perspective.

The PR does assume that we're going to elsewhere in the spec define "standard layout", but I expect that to basically match C++'s definition with the additional constraint that a standard layout type cannot be intangible.

[s-perron]

I think this is reasonable. As for the layout of the struct, saying they cannot be in constant buffers solves most of the layout issues. There is now only one layout that is used. It can change for SPIR-V with a compiler options, but the layout is still well defined by the spec.

The only concern I have is the SPIR-V implementation for function scope unions. Function scope variables in SPIR-V do not have an explicit layout. That can make an implementation awkward, but that is a SPIR-V issue. From an HLSL spec perspective this is good.

[llvm-beanz]

Tagging @farzonl for review.