[clang] [NFC] [clang] Use std::string instead of StringRef to reduce stack usage (PR #114285)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> That sounds like MSVC's problem to solve. Why does the amount of stack size matter

Yeah, I'd like to know what the benefit is -- `StringRef` seems like the correct type to use, not `std::string`, IMO. Is this showing up in a performance profile or something?

https://github.com/llvm/llvm-project/pull/114285