Audit std::string usage in F18 and consider migrating to LLVM alternative

[RichBarton-Arm]

As part of making the F18 codebase more like LLVM, we should audit our use of std::string and try and bring ourselves closer in alignment with LLVM guidelines

[sscalpone]

My reading of the guideline is to use std::string except when increasing the size of a string & to prefer twine over concatenating.