Swift: support `BUILD_RPATH` properties

Enable passing a RPATH to Swift shared libraries.  This enables testing
libraries before they have been installed.
34 jobs for swift-rpath
Status Name Job ID 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

00:18:40

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

00:45:05

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

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

00:18:27

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-debug-ninja

00:13:18

passed cmake-moonshade-osx-release-makefiles

00:11:57

passed cmake-moonshade-osx-release-makefiles

passed cmake-terfin-windows-release-vs2019

passed cmake-terfin-windows-release-vs2019

00:36:06

passed ghostflow-stager

passed ghostflow-stager

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passed ghostflow-stager

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passed ghostflow-stager

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