[clang-tools-extra] [clangd] Support symbolTags for document symbol (PR #113669)

[Dietrich Travkin via cfe-commits]

travkin79 wrote:

I thought about the _Constant_ tag. In Java for example, there is no `const` keyword, instead the `final` keyword is used for variables to declare them as read-only. The keyword `final` means something else for methods (they cannot be overridden). 
 The keywords and their meaning seem to be very language-specific. Thus, I prefer removing the _Constant_ tag completely from the LSP PR. I think, the _Read-only_ tag make much more sense, since it is more generic, and expresses better on a higher abstraction level what we mean.

https://github.com/llvm/llvm-project/pull/113669