Disallow LTO with a preference to dynamic linking

[alexcrichton]

Closes #11154

[klutzy]

It may be better if -Z lto -Z prefer-dynamic is disallowed at driver-level to save build time.

[pcwalton]

Agreed with @klutzy.

[alexcrichton]

I'm trying to keep things like LTO and output format to be fairly self-contained instead of spraying various bits and pieces across modules. It looks like I could easily add the check if sess.lto() && sess.prefer_dynamic() { fatal } to driver/driver.rs early in the compilation process, but then you still have the error which is generated if you attempt to produce a dylib output with LTO enabled.

I could move both of these checks up much earlier in the compilation process (doesn't really make sense to me to have only one early up), but I'm afraid that it's sharding the logic of LTO too much across the compiler. It is sad that it's so late in the compilation process, but this isn't an error which anyone will hit on a regular basis. Instead, this will just be a thing you hit after you waited awhile and then you'll probably never do it again, so I don't think that it's too crucial that you see this error as soon as possible.

[ghost]

The earliest possible option of informing the user is -Z help.