complete the tests for Compiler.jl as the stdlib. (#56648)
With JuliaLang/julia#56632, Compiler.jl as the stdlib can now be tested.
However, the PR was incomplete, and when tests are actually run on
`Compiler`, which is `!== Base.Compiler`, various errors occur,
including issues caused by JuliaLang/julia#56647.
This commit resolves all these issues:
- manage the code for loading `Compiler` in `setup_Compiler.jl`,
ensuring that the stdlib version of `Compiler` is loaded when `@activate
Compiler` is used beforehand
- replace `Base.IRShow` with `Compiler.IRShow`
- test `Base.Compiler.return_type` instead of `Compiler.return_type`
This was split off from JuliaLang/julia#56636.
