[lldb] Let OptionValueRegex::Clear set to value to the default and not an empty regex

Since D75537 the test suite clears all settings before a test. This caused
two tests to fail:
    lldb-api :: functionalities/inline-stepping/TestInlineStepping.py
    lldb-api :: lang/cpp/std-function-step-into-callable/TestStdFunctionStepIntoCallable.py
The reason for that is that OptionValueRegex::Clear was setting the regex
to empty instead of the default value that was passed initially. This caused
that the target.process.thread.step-avoid-regexp setting which is used in the
tests was set to "" instead of "^std::".

This patch is just a quick fix that sets the regex back to the original value
to make the tests pass.

In total these 3 setting values have changed with D75537 and also need to be
fixed (even though they don't seem to break any tests).
  target.process.thread.step-avoid-regexp (regex) -> from '^std::' to empty string
  platform.module-cache-directory (file) -> from "~/.lldb/module_cache" to empty string
  script-lang (enum) -> from 'default' to 'python'