LTOModule::isBitcodeFile() shouldn't assert when returning false.

Fixes a bunch of assert-on-invalid-bitcode regressions after 315483.
Expected<> calls assertIsChecked() in its dtor, and operator bool() only calls
setChecked() if there's no error. So for functions that don't return an error
itself, the Expected<> version needs explicit code to disarm the error that the
ErrorOr<> code didn't need.

https://reviews.llvm.org/D39437

llvm-svn: 317010