Include the following changes:
- automatically find Rust sources through rustup;
- do not return canonical type if identical to type in C-family languages;
- improve support of system Boost and system libclang on Gentoo;
- improve Red Hat and CentOS detection;
- consider header file entries in compilation database;
- improve completion of include statements in C-family languages;
- rename completer options in installation script;
- only include one macOS toolchain.
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