[clangd] Add support for additional symbol tags proposed for LSP 3.18 (#167536)
Implements support for symbol tags proposed for LSP 3.18 in
https://github.com/microsoft/language-server-protocol/pull/2003,
in the `documentSymbols` and `workspace/symbols` requests.
Fixes https://github.com/clangd/clangd/issues/2123.
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Co-authored-by: chouzz <zhouhua258@outlook.com>
Co-authored-by: Dimitri Ratz <dimitri.ratz@thinkdigital.cc>