Merges most (but not all) of the impeller .clang-tidy rules into the
main .clang-tidy config. Merges:
readability-identifier-naming.PrivateMemberSuffix (_)
readability-identifier-naming.EnumConstantPrefix (k)
modernize-use-default-member-init.UseAssignment
Does not merge:
readability-identifier-naming.PublicMethodCase (CamelCase)
readability-identifier-naming.PrivateMethodCase (CamelCase)
These last two are not merged due to the non-trivial number of existing
field accessors that use field_name() methods to directly return
field_name_. While these are permitted by the C++ style guide, we may
want to move to a single, simple rule and name everything in CamelCase.
These can be enabled in a followup patch.
No new tests added, since this change is style-only.