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/MachineIndependent/preprocessor/PpContext.h

@@ -111,6 +111,7 @@ public:
     bool   space;  // true if a space (for white space or a removed comment) should also be recognized, in front of the token returned
     int    ival;
     double dval;
+    long long i64val;
     int    atom;
     char   name[MaxTokenLength + 1];
 };