Lack of error from language service masks build error in error list

[cartermp]

Use #6035, noting that there is nothing in the Error List by default (Build + IntelliSense).

Now build the project, noting that it fails. Look at the error list:

Now switch to "Build only":

The expectation is that for Build + IntelliSense, you'd see the FS0247 error.

[dsyme]

This is actually a pretty significant bug. Even in F# 2.0 a failure in correct diagnostic list unification between intellisense and build diagnostics would have been considered a high-priority bug.

I think I've seen often too (Since about VS2015 I've learned not to trust the "Build + Intellisense" to show me build errors)