Error on cbuffer variable initializer

[llvm-beanz]

Is your feature request related to a problem? Please describe.
For 9 years DXC has silently ignored initializers on cbuffer values. It's time to make that an error.

Describe the solution you'd like
If a shader initializes a cbuffer member or other global constant that gets folded into a cbuffer the compiler should emit an error.

Describe alternatives you've considered
We could make it a warning, but since we really do nothing with the value IMO it's better to error to force users to deal with it.

Additional context
Related issue: microsoft/DirectXShaderCompiler#2384