This implements the SPV side of the GLSL commit 033ae528.

This reverts commit 2c65069e, reversing
changes made to fa9b465b.

Legalization: Omit legalization warning when spirv-opt is linked.

Update spirv-tools known_good

Add more error checks for interpolateAtVertexAMD()

Still missing is automation and final decisions on mechanisms in general.

This was done on GitHub, but lots of new code did not change when
doing the merge.

Implement the extension GL_AMD_gpu_shader_half_float_fetch

Fix issues of SPIRV headers

- Support new opaque types: f16sampler*, f16image*, f16subpassInput*.
- Add new built-in GLSL texture/image functions.

These are conditional now on which target environment is requested.

Note the tests do not yet generate the correct code regarding what extensions
they declare.