Replace --swiftcopt with starlark flag

[keith]

The Swift configuration fragment is being removed from bazel, this adds
the starlark flags necessary to replace the functionality. This differs
from the upstream solution since google doesn't have a need to support a
--host_swiftcopt replacement

(cherry picked from commit 8be7247)

[keith]

This can be used with --flag_alias to make the calling syntax more concise:

  --flag_alias=host_scopt=@build_bazel_rules_swift//swift:exec_copt
  --flag_alias=scopt=@build_bazel_rules_swift//swift:copt

Looks to me like it's safe to make those aliases swiftcopt and host_swiftcopt to mirror the flags that exist today, but that might be too confusing until those are gone for good from bazel

[brentleyjones]

95f49c9

[keith]

added that one here