Ensure proper handling of a line-break following an incomplete string interpolation.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems?id=405887.
The error recovery wasn’t including the line-break into interpolated string, yet wasn’t creating proper statement terminator token for it. The change ensures that the line-break is treated as part of the string for the purpose of the error recovery, this is what would happen if the interpolation was properly terminated with ‘}’ before the end of the line.

Fixes #18188

Removes control type from NavBar's accessible names

Report an NFW instead of asserting when we encounter a symbol we can't roundtrip to OOP.

Don't offer to convert an 'if' to a 'throw expressoin' if there is an 'else clause'

Add back the PR template.

Include default in 'Compiler Test Plan.md'

This change fixes two problems in our powershell scripts

1. Removes all uses of Invoke-Expression.  This commandlet doesn't do error checking on windows
commands and hence can hide failures.  Relpaced with Exec-Expression which properly checks
both the windows command error and the powershell error
1. Reset $LastExitCode inside Exec before executing the script block.  This avoids cases where
previous windows command failures could carry over to a script block that didn't execute any
windows commands.

Powershell guidelines changed appropriately

https://github.com/dotnet/roslyn/blob/master/docs/contributing/Powershell%20Guidelines.md

Fix extract method test baseline

Fix CsharpCodeActions.FastDoubleInvoke flakiness

Do not fully qualify type name if it would not resolve error.

Classify type names, even when arity does not match.

Port BasicEndConstruct

Fix crash in implements clause completion

Fix Winforms DeleteControl tests

Merge dev15.2.x to master

Provide completion that offers the explicit-interface-type when typing new members.

This change retargets the CSharpSymbolTest project from .NET 4.6 (which runs only on the desktop CLR) to netstandard 1.3 (which runs on both the Desktop 4.6 framework and CoreCLR).

These changes are necessary to enable more test coverage on CoreCLR, which will be especially impactful when we want to test language features which are only targeted to run on CoreCLR.

There are a lot of file changes in this PR but the most significant new code is in the TestExecutionContext, which is a new CoreCLR AppContext-based mechanism for running in-memory assemblies. This class is meant to have similar functionality to the existing desktop RuntimeEnvironment, but because AppDomains and AppContexts are incompatible, there are some substantial differences. Most notably, the TestExecutionContext views all assemblies next to the CoreCLR test runner (the "app base") as being shared among all test execution runs, including static variables. In contrast, the AppDomain runner only shares assemblies which are already loaded.

The second notable change is a rename from CreateCompilationWithMscorlib to CreateStandardCompilation. Despite the name, the most common usage of CreateCompilationWithMscorlib was just meant to create a simple compilation. The .NET Framework mscorlib was provided as a reference because it provided most of the common types our tests need to compile against, but it wasn't particularly important which types it supplied, beyond a minimum set.

Unfortunately, compiling against the .NET Framework mscorlib can introduce severe incompatibilities when trying to run the application on CoreCLR (which uses System.Runtime as its basis). This PR instead provides a new set of references (NETStandard.Library 2.0, the "standard" references) to try to provide as much compatibility as possible while also providing portability. Thus, the CreateCompilationWithMscorlib methods were renamed to CreateStandardCompilation to make it clear that no specific set of references is guaranteed when compiling against it. If that is desired, it's recommended to use the existing CreateCompilation methods to construct an explicit set of references to compile against (with all the implications that come from that).

Enable use of Sqlite in the OOP process and move our features over to using them.