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.

SPIRV/SpvBuilder.h

@@ -191,6 +191,9 @@ public:
     Id makeUint64Constant(unsigned long long u, bool specConstant = false)  { return makeInt64Constant(makeUintType(64),                     u, specConstant); }
     Id makeFloatConstant(float f, bool specConstant = false);
     Id makeDoubleConstant(double d, bool specConstant = false);
+#ifdef AMD_EXTENSIONS
+    Id makeFloat16Constant(float f16, bool specConstant = false);
+#endif
 
     // Turn the array of constants into a proper spv constant of the requested type.
     Id makeCompositeConstant(Id type, std::vector<Id>& comps, bool specConst = false);