[build-script] Add --install-back-deploy-concurrency.

Allow the installation of the back-deployment shared libraries for
concurrency via build-script.

Author: DougGregor

utils/build_swift/build_swift/driver_arguments.py

@@ -599,6 +599,9 @@ def create_argument_parser():
     option(['--back-deploy-concurrency'], toggle_true('build_backdeployconcurrency'),
            help='build back-deployment support for concurrency')
 
+    option(['--install-back-deploy-concurrency'], toggle_true('install_backdeployconcurrency'),
+           help='install back-deployment support libraries for concurrency')
+
     option(['--libcxx'], toggle_true('build_libcxx'),
            help='build libcxx')
 

utils/build_swift/tests/expected_options.py

@@ -476,6 +476,7 @@ class BuildScriptImplOption(_BaseOption):
               dest='test_early_swift_driver', value=False),
 
     SetTrueOption('--back-deploy-concurrency', dest='build_backdeployconcurrency'),
+    SetTrueOption('--install-back-deploy-concurrency', dest='install_backdeployconcurrency'),
     SetTrueOption('--benchmark'),
     SetTrueOption('--clean'),
     SetTrueOption('--dry-run'),

utils/swift_build_support/swift_build_support/products/backdeployconcurrency.py

@@ -137,7 +137,7 @@ def test(self, host_target):
         raise RuntimeError("Testing not implemented")
 
     def should_install(self, host_target):
-        return False
+        return self.args.install_backdeployconcurrency
 
     def install(self, host_target):
         destdir = self.host_install_destdir(host_target) + self.args.install_prefix