contains_lower doesnt exist llvm13
With llvm they renamed StringRef's method names and removed old ones [1] and [2]
cause build break on llvm13 systems.
contains_lower -> (probably to) contains_insensitive
It will be nice to add a control for which version of llvm is using while compiling and use that version's code for contains_.... part
[1] : https://reviews.llvm.org/D104820
[2] : https://github.com/llvm/llvm-project/commit/2e4a2b8430aca6f7aef8100a5ff81ca0328d03f9