[clang] [clang-tools-extra] [clangd] Add InsertReplaceEdit for code completion (PR #187623)

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argothiel wrote:

Thank you, @timon-ul. I'll work on the other issue. The whitespace is a bit tricky to decide, I can imagine that in `myfun^ <T>(x, y)`, it makes sense to look past the whitespace to decide about skipping the parens. But I think it's a discussion for another PR.

https://github.com/llvm/llvm-project/pull/187623