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

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.