* Significantly expanded testing.
* Reworded exception messages.
* Simplified handling of getting semantic information in the CSharpSemanticModel.
* Added comments as appropriate.
* Added linux support for running the IOperation test hook from cli.

This change skips executing analyzers by default on explicit builds (both inside VS and on command line prompt). This should speed up the overall build time of Roslyn.sln when building locally.

The only time a developer would want analyzers to run locally outside the typing scenario is to compute the exact set of warnings for a specific project/solution before pushing your local changes to CI. They can now perform the following steps to achieve this:
1. On command line: Build with `/p:UseRoslynAnalyzers=true` or `/p:RunCodeAnalysis=true`
2. Inside VS:
   1. Start VS with `%RoslynEnforceCodeStyle% = true`, all builds in VS should execute analyzers
   2. Use `Run Code Analysis` command from `Analyze` menu on selected project/solution

Co-authored-by: NFred Silberberg <fred@silberberg.xyz>

Change Roslyn to target netcoreapp3.1 when building .NET Core assets.

Previously the code targetted a mix of netcoreapp2.1 and netcoreapp3.0.
The mix is due to default interfaces only being supported on
netcoreapp3.0 and hence our testing needed to use that. Yet at the same
time we were required to ship the compiler in SDKS that targetted
netcoreapp2.1. Now we can universally target netcoreapp3.1.

Support building the workspaces layer in source-build. Also added a verification step to CI to ensure we don't break the build.

- Remove invoke mono script
 - Pass through debug flag via supported arcade mechanism instead

Generate a binary log file for the bootstrap phase on Unix platforms.

The next arcade merge will be setting this centrally.

This cleans up all of the PROTOTYPE comments around using the 3.0 SDK
inside the DIM branch.

Enable to run apps targeting netcoreapp2.1 against netcoreapp3.0
Run tests targeting netcoreapp3.0 on Linux CoreClr

Also re-enable docker caching

* Mark eng\docker\mono.sh executable

* Report error if docker build fails.

* Remove RuntimeIdentifier specification

* Arcade directory layout

* Use Arcade testing targets

* Skip CodeStyle tests on Mono

* Include net472 in test target frameworks for Mono

* Remove incorrect file existence check.

* Update ADO integration test yaml

* Use Arcade dotnet install scripts

* Update roslyn-tools build tasks to 1.0.0-beta3.18523.2

* DotNet version check, remove LocateDotNet.

Move Linux build logic to build\scripts\build.sh
Eliminate MicroBuild scripts