Emit ODR related diagnostic only from ASTStructuralEquivalence check

[balazske]

Currently at ODR failure some of the diagnostic messages are put from ASTImporter, some of them from the IsStructurallyEquivalent functions. All of these warnings should be moved into the structurally equivalence checks. There is more information about the exact type of the problem and the problem is there detected first (if a variable is not equivalent with another it may be because its type is not detected as equivalent but the warning is still put out for the variable). There are many places where no warning is produced at all, these should be corrected (get warnings for all cases when IsStructurallyEquivalent returns false not after calling another IsStructuralyEquivalent).

[balazske]

It is not possible to fully achieve this task (without too big changes):

• For functions no messages can be produced from inside structural equivalence check. The structural equivalence failure can happen because overloading or because real ODR problem but it is not possible to decide what happens.
• For detected name differences no warning or error can be produced because this should be not an ODR problem. But in some cases this may be ODR problem, for example at different named fields in a class (but not if the field is checked in itself, not as part of a class).
• Many new kinds of ODR failure messages must be added if we want precise error information.
• It is not obvious at what place an ODR error must be produced, if a check fails that is part of another check.
• ASTImporter has the knowledge about what is checked (on top level) and for what purpose (and the generated message may depend on this) but IsStructurallyEquivalent has the knowledge about what difference was exactly detected.
• Functions in ASTStructuralEquivalence.cpp are poorly designed, some things are checked at multiple places. Some refactoring may be needed here to have the correct (and one) place where a message for a specific found difference is generated.