IR: support global variable with initializers (#294)

The big change here is that the ability to contain basic blocks with instructions in them has been hoisted from `IRFunc` into a new base type `IRGlobalValueWithCode` shared with `IRGlobalVar`.
The basic blocks of a global variable define initialization logic for it; they can be looked at like a function that returns the initial value.

Places in the IR that used to assume functions contain all the code need to be updated, but so far I only handled the cloning step.

The emit logic currently handles an initializer for a global variable by outputting its logic as a separate function, and then having the variable call that function to initialize itself. This should be cleaned up over time so that we generate an ordinary expression whenever possible.

I also made the emit logic correctly label any global variable without a layout (that is, any that don't represent a shader parameter) as `static` so that the downstream HLSL compiler sees them as variables rather than parameters.

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