Expose metadata to mark analyzers for concurrent execution

[mavasani]

[NOTE: Split from https://github.com//issues/1565, which now only tracks enabling incremental analysis for 3rd party analyzers in IDE]

This issue tracks exposing metadata from analyzers to mark them as thread-safe, such that analyzer driver will be free to make concurrent action callbacks into the analyzer instance.

[mavasani]

Possible design approaches:

1. Expose attribute which can be applied on DiagnosticAnalyzer.
2. Expose a property on DiagnosticDescriptor
3. Expose an API on AnalysisContext passed into Initialize method.

[sharwell]

This is a big deal for performance in large projects. 👍 I anticipate a majority of analyzers either supporting this from the start or only requiring minor updates to work with it.

[mavasani]

Design team decided on approach 3.
A new API AnalysisContext.RegisterConcurrentExecution() will be added - analyzers which invoke this API might have their unrelated actions (basically all the non-start and non-end actions) invoked concurrently by the analyzer driver.

[menees]

The VS 2015 Update 2 announcement references this issue as "enabling Analyzer writers to mark their analyzers for concurrent execution". But it took me some digging to discover that the new API is not AnalysisContext.RegisterConcurrentExecution() as mentioned by mavasani on Dec 1 2015. The new API is actually AnalysisContext.EnableConcurrentExecution(). I wanted to point that out here to save other people some time. Currently, RegisterConcurrentExecution is only referenced in a cref comment link in roslyn-master\src\Test\Utilities\Desktop\CommonDiagnosticAnalyzers.cs, and that probably should be changed to EnableConcurrentExecution.

The Update 2 announcement also mentions "providing control over whether analyzers run in generated code". For lack of a better place to put it, here's an example of doing that: analysisContext.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);