Enable existing "strict" warnings under /warnversion

[gafter]

This depends on the completion of #45702. It is part of #45701. This is an umbrella issue to track the work of enabling existing diagnostics that are reported under "strict" mode in warning wave "5". See https://github.com/dotnet/roslyn/projects/10#column-3494327, possibly including the following:

• Enable CS8073 in a warning wave #45744: CS8073: warn when expression is always false or true
• CS0185: do not allow locks on non-reference types
• CS7023: do not allow as or is on static types
• [Strict] Do not ignore private reference fields of structs for definite assignment #30194 CS0165: track definite assignment of structs across assemblies
• CS0019: additional error for type conversions
• CS0149: Do not accept refs to delegate expressions
• [Strict] Do not ignore private reference fields of structs for definite assignment #30194 CS0177: Do not ignore private instance fields of structs imported through metadata
• Review the compiler for any further diagnoistics reported in "strict" mode.

Note: Most of these suggestions are errors rather than warnings when diagnosed in strict mode. We will need to decide whether to downgrade them to warnings (presumably with a new error code) when reporting them in a warning wave, or to permit errors to be reported in a wave.

[gafter]

Some of these are targeting 16.8. Others will be deferred. When we schedule or start work on one, we'll create a work item to track it.