[clang] [clang-tools-extra] [clang][clangd] Resolve decltype when printing types for display (PR #223950)

[Christian Kandeler via cfe-commits]

ckandeler wrote:

Note for reviewers: the code-completion/signature-help half of this is separable from the clangd hover cleanup. If you would rather take only the latter — which just removes a workaround and closes the two FIXMEs — I am happy to split it into two commits.

https://github.com/llvm/llvm-project/pull/223950