[test] Add new libdispatch -vfsoverlay flag for linux, as in swiftlang/swift-corelibs-libdispatch#785

This allows the tests that use libdispatch to find its modulemap, plus add the
libdispatch compilation flags to one test that was missing them.

Author: finagolfin

test/Concurrency/Runtime/custom_executors_complex_equality_crash.swift

@@ -1,4 +1,4 @@
-// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -parse-as-library)
+// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking %import-libdispatch -parse-as-library)
 
 // REQUIRES: concurrency
 // REQUIRES: executable_test

test/lit.cfg

@@ -1643,15 +1643,17 @@ elif (run_os in ['linux-gnu', 'linux-gnueabihf', 'freebsd', 'openbsd', 'windows-
     libdispatch_artifact_dir = config.libdispatch_build_path
     libdispatch_swift_module_dir = make_path(libdispatch_artifact_dir, 'src', 'swift', 'swift')
     libdispatch_source_dir = make_path(config.swift_src_root, os.pardir, 'swift-corelibs-libdispatch')
+    libdispatch_vfs_yaml =  make_path(libdispatch_artifact_dir, 'dispatch-vfs-overlay.yaml')
     libdispatch_artifacts = [
+        libdispatch_vfs_yaml,
         make_path(libdispatch_artifact_dir, 'libdispatch.so'),
         make_path(libdispatch_artifact_dir, 'libswiftDispatch.so'),
         make_path(libdispatch_swift_module_dir, 'Dispatch.swiftmodule')]
     if (all(os.path.exists(p) for p in libdispatch_artifacts)):
         config.available_features.add('libdispatch')
         config.libdispatch_artifact_dir = libdispatch_artifact_dir
-        config.import_libdispatch = ('-I %s -I %s -L %s'
-            % (libdispatch_source_dir, libdispatch_swift_module_dir, libdispatch_artifact_dir))
+        config.import_libdispatch = ('-I %s -I %s -L %s -vfsoverlay %s'
+            % (libdispatch_source_dir, libdispatch_swift_module_dir, libdispatch_artifact_dir, libdispatch_vfs_yaml))
 
     libdispatch_static_artifact_dir = os.path.join(config.libdispatch_static_build_path, 'lib')
     libdispatch_static_artifacts = [