Add support for GL_NV_explicit_typecast.

[alelenv]

GLSL spec here : KhronosGroup/GLSL#304

Glslang only change, no changes required for SPIRV Headers / Tools

[arcady-lunarg]

Since the spec specifically calls out (vec3) (1, 2, 3) as something that's likely to give undesirable results, would it be possible to warn in that case? That seems like it would be a good way to let users know that their code is probably not doing what they want.