Handle diagnostics through #if condition evaluation

Allow #if condition evaluation to produce proper diagnostics, including
warnings, while performing the evaluation. This covers both
warnings/errors directly coming from #if handling and also those
thrown as errors by the build configuration.

Introduce assertion functions to make it a lot easier to properly test
the tests with calls to these assertions.

Add the remaining diagnostics that were FIXME'd because we couldn't
report them properly before.

Author: DougGregor