gn build: Fix `lld-link: unknown flag: -fuse-ld=lld` warnings on Windows

Fixes a minor regression from r351248.

While here, also make it possible to opt out of lld by saying
use_lld=false when clang_base_path is set. (use_lld still defaults to
true if clang_base_path is set.)

llvm-svn: 352415