[llvm] [llvm-readtapi] Add symlink to call `readtapi` (PR #67748)

[Cyndy Ishida via llvm-commits]

cyndyishida wrote:

> I see the usage of `add_llvm_tool_symlink` in other tools. Could that simplify the change?

For the sake of completeness, this is actually enough for my goal to install downstream. It turned out the downstream project had some defaults that didn't play nicely with llvm-tools. Specifically `set(LLVM_BUILD_TOOLS OFF CACHE BOOL "")`

https://github.com/llvm/llvm-project/pull/67748