Add Clang system config action (15/24)

[MikeMcQuaid]

Four LLVM and Clang formulae share macOS target configuration generation
for the active SDK and kernel versions.

• skip non-macOS installs and generated native configurations
• select the current or versioned Command Line Tools SDK
• emit compatible arm64, aarch64 and x86_64 target files

AI disclosure: using OpenAI Codex 5.6 Sol max with local review and
testing.

[Copilot]

Pull request overview

This PR introduces a new configure_clang_system install-step action to generate macOS Clang target configuration (.cfg) files under etc/clang, and wires it into the InstallSteps DSL, RuboCop allowlists, tests, and documentation.

Changes:

• Add Utils::Clang.write_system_config_files to emit per-target .cfg files pointing at the CLT SDK sysroot.
• Add a new InstallSteps DSL action (configure_clang_system) plus Runner dispatch and a corresponding formula action implementation.
• Update RuboCop allowlists, add RSpec coverage, and document the new action in the Formula Cookbook.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Library/Homebrew/utils/clang.rb	New utility to write Clang target config files for multiple architectures and system/version pairs.
Library/Homebrew/test/utils/clang_spec.rb	Adds a unit spec for the new config file generator.
Library/Homebrew/test/rubocops/install_steps_spec.rb	Updates RuboCop expectations to allow configure_clang_system in steps blocks.
Library/Homebrew/test/install_steps_spec.rb	Adds a dispatch test ensuring the new DSL step routes to the Runner method.
Library/Homebrew/rubocops/shared/install_steps_helper.rb	Adds configure_clang_system to the list of allowed formula action step methods.
Library/Homebrew/install_steps/formula_actions.rb	Implements run_configure_clang_system to generate configs on macOS when needed.
Library/Homebrew/install_steps.rb	Adds the DSL method and Runner case dispatch for configure_clang_system.
docs/Formula-Cookbook.md	Documents the new action in the cookbook’s “formula action steps” list.

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