[clang] [clang-tools-extra] Remove `StringLiteral` in favor of `StringRef` (PR #122366)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

For reference, when it was added, the important thing about StringLiteral was that it had a constexpr constructor.  The StringRef constructor became constexpr with 57effbdadc0b382db4ef86addcfd03a64c9eda0a, a few years after StringLiteral was added.

https://github.com/llvm/llvm-project/pull/122366