Rollup merge of #62615 - wesleywiser:pgo_error, r=nagisa
Rollup merge of #62615 - wesleywiser:pgo_error, r=nagisa

 Only error about MSVC + PGO + unwind if we're generating code

In #61853, we changed the error when using PGO & MSVC toolchain & panic=unwind into a warning. However, in the compiler team meeting on 2019-07-11, we found that not everybody was in favor of that change because of the possibility of broken binaries.

This PR reverts that change so this is again an error. However, to work around an [issue the Firefox team is having](https://github.com/rust-lang/rust/issues/61002#issuecomment-500075739), we will only emit the error if we're actually supposed to generate a binary. If the `rustc` is invoked with `--print` arguments (which means that no binary will actually be emitted), then we won't emit the error because there is not a possibility of the issue occurring.

cc @EricRahm @nikomatsakis @pnkfelix @Centril

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust