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/preprocessor/PpTokens.h

@@ -123,6 +123,9 @@ enum EFixedAtoms {
     PpAtomConstUint64,
     PpAtomConstFloat,
     PpAtomConstDouble,
+#ifdef AMD_EXTENSIONS
+    PpAtomConstFloat16,
+#endif
     PpAtomConstString,
 
     // Identifiers