add build option for LLVM debug info

[mrhota]

See relevant discussion in #31033. We probably want control of LLVM debuginfo independent of --disable-optimize-llvm. That's currently the only way to get debug symbols in LLVM (that I know of).

I suggested in the above issue that we call the option --enable-llvm-debuginfo.

CC @nnethercote @nagisa

[mrhota]

cc @alexcrichton A-rustbuild, A-build, and A-debuginfo seem like relevant tags