[clang-format] [PR19056] Add support for access modifiers indentation

Adds support for coding styles that make a separate indentation level for access modifiers, such as Code::Blocks or QtCreator.

The new option, `IndentAccessModifiers`, if enabled, forces the content inside classes, structs and unions (“records”) to be indented twice while removing a level for access modifiers. The value of `AccessModifierOffset` is disregarded in this case, aiming towards an ease of use.

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The PR (https://bugs.llvm.org/show_bug.cgi?id=19056) had an implementation attempt by @MyDeveloperDay already (https://reviews.llvm.org/D60225) but I've decided to start from scratch. They differ in functionality, chosen approaches, and even the option name. The code tries to re-use the existing functionality to achieve this behavior, limiting possibility of breaking something else.

Reviewed By: MyDeveloperDay, curdeius, HazardyKnusperkeks

Differential Revision: https://reviews.llvm.org/D94661