test: make target_build_swift_dylib work on Windows

Use %s instead of %r for proper expansion, the substitution is not a
single argument.  Use SubstituteCaptures as there is a backreference
involved here.

Author: compnerd

test/lit.cfg

@@ -823,7 +823,9 @@ elif run_os in ['windows-msvc']:
     subst_target_swift_frontend_mock_sdk_after = ''
 
     config.target_build_swift_dylib =                                            \
-            ("%r -parse-as-library -emit-library -o '\\1'" % (config.target_build_swift))
+            SubstituteCaptures("%s -parse-as-library -emit-library -o \\1" % (config.target_build_swift))
+    config.target_add_rpath = r''
+
     config.target_clang =                                                        \
             ('clang++ -target %s %s' % (config.variant_triple, clang_mcp_opt))
     config.target_ld =                                                           \