[clangd] Enhance Clangd rename testing coverage

We plan to eliminate error-prone and obsolete Clang-Rename API from Clangd. To
do that, we will introduce Decl canonicalization rules that will make renaming
code simpler and easier to maintain (D71880).

To ensure smooth transition to the new implementation, many Clang-Rename tests
will be adopted in Clangd to prevent any possible regressions. This patch is
the first in the chain of test migration patches. It improves existing tests
and adopts tests from Clang-Rename's alias, class and enum testing files.

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D91102