Auto merge of #1701 - alexcrichton:lenient-rustc-flags, r=brson

Auto merge of #1701 - alexcrichton:lenient-rustc-flags, r=brson

Some flags to the compiler could cause it to stop early or not emit some files
altogether (or perhaps emit files in different locations even). Currently cargo
expects a few outputs of the compiler after `cargo rustc` is run, but this
commit alters cargo to know that when `cargo rustc` is being run that the
outputs may not exist and that's ok.

Closes #1675