Implement extension GL_AMD_gpu_shader_int16

- Add int16 types (int16_t, uint16_t, i16vec, u16vec).
- Add int16 support to GLSL operators.
- Add int16 type conversions (to int16, from int16).
- Add int16 built-in functions.

glslang/Include/BaseTypes.h

@@ -53,6 +53,10 @@ enum TBasicType {
     EbtUint,
     EbtInt64,
     EbtUint64,
+#ifdef AMD_EXTENSIONS
+    EbtInt16,
+    EbtUint16,
+#endif
     EbtBool,
     EbtAtomicUint,
     EbtSampler,