[build] For CI, make it a hard error if libdispatch is not available

When the `SWIFT_CI` variable is present in the environment, then we want
to enforce that the libdispatch library is available.  This should
prevent us from regressing.

Note: we also need to double-check that Swift builders set this
environment variable.

Author: Julian Lettner

test/lit.cfg

@@ -1032,6 +1032,10 @@ elif (run_os in ['linux-gnu', 'linux-gnueabihf', 'freebsd', 'windows-cygnus', 'w
         libdispatch_source_dir = make_path(config.swift_src_root, os.pardir, 'swift-corelibs-libdispatch')
         config.import_libdispatch = ('-I %s -I %s -L %s'
             % (libdispatch_source_dir, libdispatch_swift_module_dir, libdispatch_artifact_dir))
+    elif 'SWIFT_CI' in os.environ:
+        print("ERROR: could not find libdispatch build artifacts")
+        print(libdispatch_artifacts)
+        sys.exit(1)
 
     config.target_build_swift = (
         '%s -target %s -toolchain-stdlib-rpath %s %s %s %s %s'