Fix find-llvm-config to ignore LLVM_CONFIG's escape sequences

[HertzDevil]

If the LLVM_CONFIG environment variable is already set and points to a valid executable, percent signs and backslashes will be specially interpreted according to printf's rules. This is most likely to happen on MSYS2, since the Windows host might have set LLVM_CONFIG to support static LLVM builds:

$ LLVM_CONFIG='C:\Users\nicet\llvm\18\bin\llvm-config.exe' src/llvm/ext/find-llvm-config
src/llvm/ext/find-llvm-config: line 19: printf: missing unicode digit for \U
C:\Users
icet\llvmin\llvm-config.exe

This PR ensures that doesn't happen.