Swift: support `INSTALL_NAME_DIR` on Darwin

Adjust the shared object rule to ensure that we honour the
`INSTALL_NAME_DIR` property on Swift targets.  This enables the use of
`INSTALL_NAME_DIR` and `BUILD_WITH_INSTALL_NAME_DIR` on Darwin with
Ninja.
5 jobs for darwin-install-name
Status Name Job ID Coverage
  External
passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed ghostflow-check-master

passed ghostflow-stager