[caffe2] constexpr const
Summary:
To fix warning:
```
xplat\\caffe2\\torch\\csrc\\jit\\runtime\\instruction.cpp(59,20): warning: ISO C++11 does not allow conversion from string literal to 'char *const' [-Wwritable-strings]
```
which can be seen in Windows CI logs.
Test Plan: Eyes; did not run it.
Reviewed By: iseeyuan
Differential Revision: D27717057
fbshipit-source-id: f365405663b5adfbc0c87dc26a9921b6d03f1f5a
