[llvm] Revert "[ADT] Use std::string_view inside StringRef (#113775)" (PR #114133)

[Jakub Kuderski via llvm-commits]

kuhar wrote:

@kazutakahirata 

> Some sanitizer failures have been reported, indicating that StringRef
and std::string_view handle data == nulptr differently. Also, they
support different values for the max size (size_t v.s. ptrdiff_t).

We should pre-commit unit tests that cover these differences 

https://github.com/llvm/llvm-project/pull/114133