gn build: Merge r358749

Since the symlinks list for llvm-symbolizer is now never empty,
the :symlinks target no longer needs an explicit dep on :llvm-symbolizer
-- there will be at least one dep on a symlink, and each symlink depends
on :llvm-symbolizer already.

Since llvm-symbolizer:symlinks now produces symlinks that check-llvm
uses, make llvm/test depend on the symlink target.

llvm-svn: 358861