[clangd] Make it possible to enable misc-const-correctness clang-tidy check (#94920)

Before this PR, clangd forcefully disabled misc-const-correctness in
disableUnusableChecks().

Now we have a FastCheckFilter configuration whose default value
(Strict) also disables it. This patch removes misc-const-correctness
from disableUnusableChecks() so it's possible to enable by setting
FastCheckFilter to None.

Fixes https://github.com/llvm/llvm-project/issues/89758