After introducing implicit types via
https://github.com/eclipse-jdt/eclipse.jdt.core/pull/2066 the snippet
that contains only main() method is recognized as
ImplicitTypeDeclaration that does not inherit AbstractTypeDeclaration
but common super class AbstractUnnamedTypeDeclaration.

Make sure we don't fail with CCE and don't generate extra main method
that wraps around pasted main method.

Fixes https://github.com/eclipse-jdt/eclipse.jdt.ui/issues/1414