Add -fsanitize=leak to driver options.

If -fsanitize=leak is specified, link the program with the
LeakSanitizer runtime. Ignore this option when -fsanitize=address is specified,
because AddressSanitizer has this functionality built in.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182729 91177308-0d34-0410-b5e6-96231b3b80d8