Validation tests don't define llvm_plugin_ext

This caused a Windows-specific error in https://github.com/llvm/llvm-project/blob/2f7d3ec02349d4503e1fa8c54665025eb4f8e173/llvm/utils/lit/lit/TestRunner.py#L1790

Author: weliveindetail

test/lit.cfg

@@ -239,7 +239,9 @@ elif platform.system() == 'Darwin':
 config.substitutions.append( ('%lto_flags', lto_flags) )
 config.substitutions.append( ('%use_just_built_liblto', use_just_built_liblto) )
 config.substitutions.append( ('%llvm_libs_dir', llvm_libs_dir) )
-config.substitutions.append( ('%llvm_plugin_ext', llvm_plugin_ext) )
+
+if llvm_plugin_ext is not None:
+    config.substitutions.append( ('%llvm_plugin_ext', llvm_plugin_ext) )
 
 # Allow tests to restore the original environment if they need to.
 config.substitutions.append( ('%original_path_env', config.environment['PATH']) )