HLSL Raytracing shader library support

It would be nice to be able to compile shader library like in [DirectXShaderCompiler](https://github.com/microsoft/DirectXShaderCompiler).
Or at least have a way for enumeration entrypoints from library.
For get:
```
stage rgen, entry "ray_gen"
stage rmiss, entry "miss"
stage rchit, entry "closest"
```
From:
```
[shader("raygeneration")]
void ray_gen()

[shader("miss")]
void miss(inout RayPayload payload)

[shader("closesthit")]
void closest(inout RayPayload payload, in BuiltInTriangleIntersectionAttributes attribs)
```

The issue names no files or tests. Start by reviewing glslang's HLSL front end and comparing the requested shader-library behavior with DirectXShaderCompiler. Done means supporting shader-library compilation or enumerating the raytracing entry points shown in the examples.

Written by the indexing model from the issue text.