build: reserve `--color-diagnostics` for lld

When building out-of-tree compilers (e.g. swift), the linker check here
may yield incorrect values.  Ensure that we are using lld before we
attempt to use `--color-diagnostics` for the linker.  Other linkers (i.e
bfd, gold) do not support this flag and the test can pass in some cases
and then fail subsequently when building.

llvm-svn: 330158