[clang-tidy] modernize-use-override new option AllowOverrideAndFinal

[clang-tidy] modernize-use-override new option AllowOverrideAndFinal

Summary:
In addition to adding `override` wherever possible, clang-tidy's `modernize-use-override` nicely removes `virtual` when `override` or `final` is specified, and further removes override when final is specified. While this is great default behavior, when code needs to be compiled with gcc at high warning levels that include `gcc -Wsuggest-override` or `gcc -Werror=suggest-override`, clang-tidy's removal of the redundant `override` keyword causes gcc to emit a warning or error. This discrepancy / conflict has been noted by others including a comment on Stack Overflow and by Mozilla's Firefox developers.

This patch adds an AllowOverrideAndFinal option defaulting to 0 - thus preserving current behavior - that when enabled allows both `override` and `final` to co-exist, while still fixing all other issues.

The patch includes a test file verifying all combinations of virtual/override/final, and mentions the new option in the release notes.

Reviewers: alexfh, djasper, JonasToth

Patch by: poelmanc

Subscribers: JonasToth, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70165