…ings.

Introduce MSBuildTreatWarningsAsErrors property that when set to true will treat all warnings as errors when building a project.
Introduce MSBuildWarningsAsErrors property which is a semicolon delmited list of warning codes to treat as errors when building a project.
Introduce MSBuildWarningsAsMessages property which is a semicolon delimited list of warning codes to treat as low importance messages.

This allows users to control warnings at the project level via standard MSBuild properties.  They can be set in a project, an import, or from the command-line.

The limitation here is that the warnings can only include ones that occur during a build.  This is because the properties are not read until after parsing so warnings generated during parse/evaluation happen too soon.  For these warnings, users will only be able to treat them as errors/messages with the /WarnAsError and /WarnAsMessage command-line arguments.

Closes dotnet#1886

Should reduce memory use when building large project trees