[clang] [clang][builtins] Return a `SmallString` from `Builtin::Context::getQuotedName()` (PR #227963)

[Chandler Carruth via cfe-commits]

https://github.com/chandlerc approved this pull request.

The return of `SmallString` doesn't worry me with modern C++ semantics -- it should get NRVO / RVO applied, and basically initialize in-place in the caller.

LGTM with the code you have or with a tweaked version I mention if it doesn't regress performance.

https://github.com/llvm/llvm-project/pull/227963