Rollup merge of #103610 - wesleywiser:thinlto_cgu1, r=michaelwoerister

Rollup merge of #103610 - wesleywiser:thinlto_cgu1, r=michaelwoerister

Allow use of `-Clto=thin` with `-Ccodegen-units=1` in general

The current logic to ignore ThinLTO when `-Ccodegen-units=1` makes sense for local ThinLTO but even in this scenario, a user may still want (non-local) ThinLTO for the purpose of optimizing dependencies into the final crate which is being compiled with 1 CGU.

The previous behavior was even more confusing because if you were generating a binary (`--emit=link`), then you would get ThinLTO but if you asked for LLVM IR or bytecode, then it would silently change to using regular LTO.

With this change, we only override the defaults for local ThinLTO if you ask for a single output such as LLVM IR or bytecode and in all other cases honor the requested LTO setting.

r? `@michaelwoerister`