[llvm] [ADT] Make StringRef std::string_view conversion operator constexpr. NFC (PR #77506)

[David Blaikie via llvm-commits]

dwblaikie wrote:

Please add some test coverage - should be easy enough to put some static_asserts in the StringRef unit test that const evaluate these operations.

https://github.com/llvm/llvm-project/pull/77506