Ensure that we use a single CompilationWithAnalyzers instance for computing diagnostics with the LatestDiagnosticsGetter (FixAll code path and other explicit GetDiagnostics requests to the diagnostic service). This avoids us cloning the compilation and re-running analyzer compilation start actions once per each document for which we need to compute diagnostics.

Background analysis based off the solution crawler still creates a new CompilationWithAnalyzers instance per document analysis request as caching it in memory causes high VM usage.

This change addresses the FixAll performance regression in VS2015 Update1 reported in https://github.com/DotNetAnalyzers/StyleCopAnalyzers/pull/1979#issuecomment-166656002.