Add support for command-line- or config-file-specified clang-tidy checks

Per https://github.com/LLNL/blt/pull/395#pullrequestreview-486204789, it would be useful to allow the user to specify `clang-tidy` checks via command-line (e.g. with a custom CMake command) or with a `clang-tidy` config file, for the purpose of applying `clang-tidy`-driven changes to source files without needing to have CMake reconfigure/regenerate.

Start with the linked pull request review for the motivation and inspect how BLT currently configures clang-tidy through CMake. Trace the existing clang-tidy integration and determine how command-line checks or a clang-tidy config file could be passed without reconfiguration. Done means users can apply those externally specified checks to source files without CMake regenerating.

Written by the indexing model from the issue text.