Revert two step initialization of constraints and base type

Revert recent changes for two step initialization of constraints and base type if those changes are no longer necessary.

Constraints: `TypeParameterSymbol.EnsureAllConstraintsAreResolved(bool early)` https://github.com/dotnet/roslyn/pull/27525
Base type: `PENamedTypeSymbol.GetDeclaredBaseType(bool ignoreNullability)` https://github.com/dotnet/roslyn/pull/28028
