Apple's clangd (/usr/bin/clangd, Xcode and CommandLineTools toolchains) is built without clang-tidy, so on a Mac without a managed clangd the checks from .clang-tidy never appear. When a clangd server for C, C++, Objective-C or Objective-C++ resolves to Apple's binary and the project has a .clang-tidy, show one warning per session with Install (managed LLVM clangd, which already takes precedence once installed) and Language Servers… actions. Fixes#692.