[lldb] Fix libcxx configuration in dotest.py (llvm#151258)

We emit a warning when running the test suite remotely that says the
libcxx arguments will be ignored, but because they're set outside the
conditional block, we're not actually do this. Fix the logic by moving
the configuration in the conditional else-block.

(cherry picked from commit db322be)

Author: JDevlieghere

lldb/packages/Python/lldbsuite/test/dotest.py

@@ -294,9 +294,11 @@ def parseOptionsAndInitTestdirs():
                 "Custom libc++ requires both --libcxx-include-dir and --libcxx-library-dir"
             )
             sys.exit(-1)
-    configuration.libcxx_include_dir = args.libcxx_include_dir
-    configuration.libcxx_include_target_dir = args.libcxx_include_target_dir
-    configuration.libcxx_library_dir = args.libcxx_library_dir
+        else:
+            configuration.libcxx_include_dir = args.libcxx_include_dir
+            configuration.libcxx_include_target_dir = args.libcxx_include_target_dir
+            configuration.libcxx_library_dir = args.libcxx_library_dir
+
     configuration.cmake_build_type = args.cmake_build_type.lower()
 
     if args.channels: