Parser: Implement extension GL_AMD_gpu_shader_half_float.

- Add built-in types: float16_t, f16vec, f16mat.
- Add support of half float constant: hf, HF.
- Extend built-in floating-point operators: +, -, *, /, ++, --, +=, -=,
  *=, /=, ==, !=, >=, <=, >, <.
- Add support of type conversions: float16_t -> XXX, XXX -> float16_t.
- Add new built-in functions.

glslang/MachineIndependent/linkValidate.cpp

@@ -950,6 +950,9 @@ int TIntermediate::getBaseAlignmentScalar(const TType& type, int& size)
     case EbtInt64:
     case EbtUint64:
     case EbtDouble:  size = 8; return 8;
+#ifdef AMD_EXTENSIONS
+    case EbtFloat16: size = 2; return 2;
+#endif
     default:         size = 4; return 4;
     }
 }