[Reland] Simplify c10::string_view implementation (#108622)
PR #108479 was reverted because
```
In file included from xplat/caffe2/c10/util/Exception.h:5:
In file included from xplat/caffe2/c10/util/StringUtil.h:6:
xplat/caffe2/c10/util/string_view.h:576:31: error: out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated [-Werror,-Wdeprecated]
basic_string_view<CharT>::npos;
```
Now this is fixed and Wdeprecated generated no warnings on my host.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108622
Approved by: https://github.com/Skylion007
