1. Add a new API: AnalysisContext.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags analysisMode)
Configures analyzer callbacks + diagnostic reporting for the analyzer. Recommended for analyzer authors to always invoke this API.

2. Analyzer driver uses a heuristic to identify generated code:
  1. Symbols marked with GeneratedCodeAttribute.
  2. Files with specific extensions (see [here](http://source.roslyn.io/Microsoft.CodeAnalysis.Workspaces/R/ef3599fb042e3706.html)).
  3. Files which have a single line comment starting with <auto-generated> at the beginning of a source file.

3. Driver defaults for non-configured analyzers:
  1. Run analysis on generated code: This avoids semantic breaks for analyzers and will be guaranteed to be always enabled in future.
  2. Report all diagnostics on generated code: This will likely be changed to perform some level of default filtering after #3705 is implemented.

Fixes #6998