[lld][clang][cmake] Clean up a few things

- If not using `llvm-config`, `LLVM_MAIN_SRC_DIR` now has a sane default

- `LLVM_CONFIG_PATH` will continue to work for LLD for back compat.

- More quoting of paths in an abundance of caution.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D118792