Add basic support for fragment shader interlock (FSI) (#1722)

Both D3D "rasterizer ordered views" (ROVs) and GLSL "fragment shader interlock" (FSI) are aimed at the same basic use case: they allow for fragment shaders to contain operations that require mutual exclusion and/or deterinistics ordering between fragment shader invocations that affect the same framebuffer coordinates. The language-level exposure of the features varies greatly between the two API families, though:

* ROVs define an implicit ordering and mutual exclusion constraint: certain resoure parameters are marked as `RasterizerOrdered`, and reads/writes to these resources must be sequences *as if* fragment-shader invocations ran in sequential order for each pixel.

* FSI defines paired begin/end functions that mark a critical section of code. All memory operations in the critical section must be sequences *as if* fragment-shader invocations ran in sequential order for each pixel. In order to make this model tractable, only a single critical section is allowed per fragment shader, and the begin/end must appear at the top level of the shader entry point function (not under control flow or after a possible conditional `return`.

The simplest way for Slang to support portable programs that run across both API families is to insist that code that cares about these ordering guarantees must use *both* mechanisms, and then each of them will only affect the API that cares about it.

Slang already supports ROV resource types, and already lowers them to plain textures for GLSL/SPIR-V.
This change adds the missing feature of a begin/end function pair for FSI, which will map to empty functions on non-GLSL targets.

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