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/Pp.cpp

@@ -705,6 +705,9 @@ int TPpContext::CPPerror(TPpToken* ppToken)
     while (token != '\n' && token != EndOfInput) {
         if (token == PpAtomConstInt   || token == PpAtomConstUint   ||
             token == PpAtomConstInt64 || token == PpAtomConstUint64 ||
+#ifdef AMD_EXTENSIONS
+            token == PpAtomConstFloat16 ||
+#endif
             token == PpAtomConstFloat || token == PpAtomConstDouble) {
                 message.append(ppToken->name);
         } else if (token == PpAtomIdentifier || token == PpAtomConstString) {
@@ -739,6 +742,9 @@ int TPpContext::CPPpragma(TPpToken* ppToken)
         case PpAtomConstUint64:
         case PpAtomConstFloat:
         case PpAtomConstDouble:
+#ifdef AMD_EXTENSIONS
+        case PpAtomConstFloat16:
+#endif
             tokens.push_back(ppToken->name);
             break;
         default: