Do not do analysis for invalid argument escape mixing when having binding errors.

Merge dev15.6.x to dev15.7.x

Port fix for duplicate IDEDiagnosticIDs to 15.7

* Fix `is` and pattern-matching behavior in presence of implicit UD conversion
and also an explicit reference conversion. User-defined conversions should
never be considered for `is` and pattern-matching.
Fixes #24522

Merge dev15.6.x to dev15.7.x

Locate implementations for reference assemblies using the GAC

Update optimization data to 2.7.0-beta3-62526-01...

Fix command line build error

* Check fully-qualified names for SuppressIldasmAttribute and ReferenceAssemblyAttribute
* Use correct reference location, or fail decompilation if it's not available

Reduce allocations in UnboundLambda

Move to xunit.console for CoreClr tests

Verify MSBuild version in Developer CMD prompt

Fix and re-enable test that would catch this error

CheckPresenceOfTypeIdentifierAttribute - avoid requesting syntax trees if we know that we already decoded all well-known attributes. (#24576)

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems?id=537870.

Remove the use of Parallel.ForEach in ClsComplianceChecker

Previously we were using xunit.console for desktop tests and dotnet-xunit for our
CoreClr tests. This change unifies us on top of xunit.console (now that it has a
netcoreapp2.0 version available).

Fix problems associated with analyzer timing

Small cleanup of completion logic.

CS1628 error text mentions in parameters; fixes #24584

Omit Default modifier style & code fix

Cleanup project system code