Implement the extension GL_ARB_gpu_shader_int64

- Add new keyword int64_t/uint64_t/i64vec/u64vec.
- Support 64-bit integer literals (dec/hex/oct).
- Support built-in operators for 64-bit integer type.
- Add implicit and explicit type conversion for 64-bit integer type.
- Add new built-in functions defined in this extension.

glslang/Include/BaseTypes.h

@@ -48,6 +48,8 @@ enum TBasicType {
     EbtDouble,
     EbtInt,
     EbtUint,
+    EbtInt64,
+    EbtUint64,
     EbtBool,
     EbtAtomicUint,
     EbtSampler,