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.

(cherry picked from commit 7589520)

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

@@ -10,6 +10,7 @@
 // rdar://76038845
 // REQUIRES: concurrency_runtime
 // UNSUPPORTED: back_deployment_runtime
+// UNSUPPORTED: back_deploy_concurrency
 
 import Darwin
 @_predatesConcurrency import Dispatch

test/lit.cfg

@@ -1804,7 +1804,8 @@ 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():
@@ -2093,6 +2094,13 @@ 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')
 
@@ -2117,7 +2125,7 @@ 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: