Add support for adding/removing suppressions for diagnostics with no source location. Additionally,  when the diagnostics being suppressed or unsuppressed are across different languages, instead of showing a preview changes dialog per-language, we now show a single preview changes dialog for the entire changed solution.

1) Add an API to the suppression fix service to create a baseline for a given project/solution
2) Don't show preview changes dialog for baseline operation.

1) Add a FixMultipleCodeAction for fixing specified set of diagnostics in a FixMultipleContext.
2) Add a IFixMultipleOccurrencesService which takes a specific set of diagnostics and computes and applies a FixMultipleCodeAction. This will eventually be used to bulk fix or suppress instances of diagnostics from the error list.

Also handle this new context and fix scope in the suppression fixers, so that the disconnected baselining scenario can use this functionality.