Preprocess flag allows some irrelevant logging to escape

```

```

```
msbuild /pp:out.txt test.proj
```
results in

```
C:\git\corefx>msbuild /pp:out.txt test.proj
The target "bt" listed in a BeforeTargets attribute at "C:\git\corefx\test.proj (3,26)" does not exist in the project,
and will be ignored.
The target "at" listed in an AfterTargets attribute at "C:\git\corefx\test.proj (3,45)" does not exist in the project,
and will be ignored.
C:\git\corefx>
```

When using `/preprocess`, no logging is expected except for cases where `/preprocess` cannot do its job (for example syntax error or import of nonexistent file)

Reproduce the issue with the XML in test.proj and `msbuild /pp:out.txt test.proj`. Trace the `/pp` or `/preprocess` logging path and determine why missing BeforeTargets and AfterTargets warnings are emitted. Done means irrelevant logging is suppressed during preprocessing while syntax errors and nonexistent imports still report failures.

Written by the indexing model from the issue text.