Add `GenericCodeGen` and `MachineIndependent` static library targets.
Privately import both of these into the `glslang` target.
Privately import `MachineIndependent` into the  `SPIRV` target.

This is done to break the dependency of `libglslang.so` non-public APIs from `libspirv.so`,
which will become problematic once `glslang` hides its non-public symbols.

| File                      | Before     |     After  |
|---------------------------|-----------:|-----------:|
| `libGenericCodeGen.a`     |  -         |   `527716` |
| `libglslang.a`            | `68175944` |   `512938` |
| `libHLSL.a`               |     `1428` |     `1428` |
| `libMachineIndependent.a` |  -         | `67132202` |
| `libOGLCompiler.a`        |    `75908` |    `75908` |
| `libOSDependent.a`        |    `23768` |    `23768` |
| `libSPIRV.a`              | `15710210` | `15710210` |
| `libSPVRemapper.a`        |  `3250894` |  `3250894` |

| File                                    | Before     |     After  |
|-----------------------------------------|-----------:|-----------:|
| `libglslang-default-resource-limits.so` |   `117032` |   `117032` |
| `libglslang.so`                         | `22380688` | `22368216` |
| `libHLSL.so`                            |     `7520` |     `7520` |
| `libOGLCompiler.a`                      |    `75908` |    `75908` |
| `libOSDependent.a`                      |    `23768` |    `23768` |
| `libSPIRV.so`                           |  `7288336` | `28151016` |
| `libSPVRemapper.so`                     |  `1940208` |  `1940208` |

Issues: #2283, #1484