glsl/types: Add support for function types

SPIR-V has a concept of a function type that's used fairly heavily.  We
could special-case function types in SPIR-V -> NIR but it's easier if we
just add support to glsl_types.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/compiler/glsl/ir_clone.cpp

@@ -373,6 +373,7 @@ ir_constant::clone(void *mem_ctx, struct hash_table *ht) const
    case GLSL_TYPE_ERROR:
    case GLSL_TYPE_SUBROUTINE:
    case GLSL_TYPE_INTERFACE:
+   case GLSL_TYPE_FUNCTION:
       assert(!"Should not get here.");
       break;
    }