Report problems/errors for all included headers

[sean-mcmanus]

In VS, all the headers in a TU show errors in the error list (with the ability to filter to only show errors for the current file). See also #3207 , which is related.

[sean-mcmanus]

This issue can make the "No IL available" harder to diagnose, see #6114.

[starball5]

What about system headers included directly or transitively? There are people who intentionally mark include directories as system include directories for the common compiler property that they don't diagnose warnings or errors from system headers.