Swift: honour `-swift-version` in Ninja generator

Swift has supported `CMAKE_Swift_LANGUAGE_VERSION` and
`Swift_LANGUAGE_VERSION` but didn't apply that to Ninja generated
targets.  Consider the property when calculating the flags.
Status Job ID Name Coverage
  External
passed cdash

passed cmake-dash3win10-windows-release-ninja

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-minoc-linux-release-lint-iwyu-tidy

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-release-makefiles

passed cmake-terfin-windows-release-vs2019

passed ghostflow-check-master

passed ghostflow-stager

passed cmake-dash3win10-windows-release-ninja

00:19:28

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:44:53

passed cmake-minoc-linux-release-lint-iwyu-tidy

00:21:32

passed cmake-moonshade-osx-debug-ninja

00:17:48

passed cmake-moonshade-osx-release-makefiles

00:12:40

passed cmake-terfin-windows-release-vs2019

00:54:48

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager