Make back-deployed concurrency library testable on OS's that have the library.

Change the order of paths provided for `DYLD_LIBRARY_PATH` when running
tests for the back-deployed concurrency library, so they supersede the
ones in `/usr/lib/swift`. This way, we can test the back-deployed
concurrency library even on OS's that have the concurrency library in
them. It's not a complete test, because the standard library and
runtime will still vary, but can reproduce many failures more easily.

When using the back-deployed concurrency library, set the lit feature
`back_deploy_concurrency`. Update some tests that are unsupported with
back-deployment to check this.

Finally, add some logging when the concurrency runtime cannot be
tested at all, to help with debugging CI in the future.

Author: DougGregor

test/Concurrency/Runtime/exclusivity.swift

@@ -6,7 +6,7 @@
 // REQUIRES: concurrency_runtime
 // UNSUPPORTED: back_deployment_runtime
 // UNSUPPORTED: OS=wasi
-// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deploy_concurrency
 
 // This test makes sure that:
 //

test/Concurrency/Runtime/exclusivity_custom_executors.swift

@@ -6,12 +6,11 @@
 // rdar://76038845
 // UNSUPPORTED: back_deployment_runtime
 // REQUIRES: concurrency_runtime
+// UNSUPPORTED: back_deploy_concurrency
 
 // Crash expectations can't be implemented on WASI/WebAssembly.
 // UNSUPPORTED: OS=wasi
 
-// UNSUPPORTED: use_os_stdlib
-
 // This test makes sure that we properly save/restore access when we
 // synchronously launch a task from a serial executor. The access from the task
 // should be merged into the already created access set while it runs and then

test/Concurrency/async_task_base_priority.swift

@@ -7,6 +7,7 @@
 // rdar://76038845
 // REQUIRES: concurrency_runtime
 // UNSUPPORTED: back_deployment_runtime
+// UNSUPPORTED: back_deploy_concurrency
 
 import StdlibUnittest
 import Dispatch

test/Concurrency/async_task_priority.swift

@@ -11,6 +11,7 @@
 // rdar://76038845
 // REQUIRES: concurrency_runtime
 // UNSUPPORTED: back_deployment_runtime
+// UNSUPPORTED: back_deploy_concurrency
 
 import Darwin
 @preconcurrency import Dispatch

test/lit.cfg

@@ -1803,7 +1803,8 @@ if back_deployment_runtime is not None:
 
 concurrency_back_deploy_path = ''
 if run_vendor == 'apple':
-    if 'back_deploy_concurrency' in config.available_features:
+    if 'back_deploy_concurrency' in lit_config.params:
+        config.available_features.add('back_deploy_concurrency')
         concurrency_back_deploy_path = os.path.join(os.path.dirname(swift_obj_root), os.path.basename(swift_obj_root).replace("swift-", "backdeployconcurrency-"), 'lib', 'swift-5.5', run_os)
 
 def os_stdlib_paths():
@@ -2092,6 +2093,13 @@ if 'concurrency' in config.available_features:
             config.available_features.add('concurrency_runtime')
         elif 'back_deploy_concurrency' in config.available_features and run_vers >= concurrency_back_deploy_version:
             config.available_features.add('concurrency_runtime')
+        elif 'back_deploy_concurrency' in config.available_features:
+            print('Disabled concurrency runtime tests because run version',
+                  run_vers, "< back deploy version",
+                  concurrency_back_deploy_version)
+        else:
+            print('Disable concurrency runtime tests because run version',
+                  run_vers, 'is too old and back-deployment is not enabled')
     else:
         config.available_features.add('concurrency_runtime')
 
@@ -2116,7 +2124,7 @@ elif not kIsWindows:
     if 'use_os_stdlib' in lit_config.params:
         config.available_features.add('use_os_stdlib')
 
-        target_stdlib_path = os_stdlib_paths() + [concurrency_back_deploy_path] + target_stdlib_path
+        target_stdlib_path = [concurrency_back_deploy_path] + os_stdlib_paths() + target_stdlib_path
 
         lit_config.note('Testing with the standard libraries in the OS')
     elif 'back_deployment_runtime' in lit_config.params: