Improve the None host

[jaredpar]

This work is the result of investigating issue #49 and causes the following changes to how the None analyzer host works.

1. The compiler log now records whether or not generated files were able to be read. This does not mean there are generated files, just that the log creation was able to successfully probe for them. This will be false when a build failed, native PDBs are used, etc ...
2. The reader now uses an in memory source generator to add back the generated files. That means every host has to go through the CompilationData.GetCompilationAfterGenerators code path. This makes consumption uniform for all the analyzer host kinds.
3. When (1) above fails then GetCompilationAfterGenerators will have a diagnostic stating this. That means consumers won't silently run into this problem.
4. Threaded through the ability to use the None analyzer host on a few of the command line options.

closes #49

work arounds for KirillOsenkov/MSBuildStructuredLog#699