[clang-tidy] Extend InheritParentConfig to CommandLineConfig

Extend the `InheritParentConfig` support introduced in D75184 for the command line option `--config`.
The current behaviour of `--config` is to when set, disable looking for `.clang-tidy` configuration files.
This new behaviour lets you set `InheritParentConfig` to true in the command line to then look for `.clang-tidy` configuration files to be merged with what's been specified on the command line.

Reviewed By: DmitryPolukhin

Differential Revision: https://reviews.llvm.org/D81949