bug#5570: automatic cache validation

[Paul Pogonyshev]

After March the 1st changes indentation in C++ and related modes is
better, but it still breaks often enough --- to the tune of once every
3--5 minutes or so.  Both when editing C++ and Java code.

As I understand, the issue is that some cached information becomes
wrong after certain (undetermined) buffer changes.  Maybe it is better
to write an automated cache validation code rather than search for a
certain way to reproduce stuff?

E.g. just build information from scratch and assert that cached
information is the same.  Ideally, such checks should be performed
after every change, so that you'd be able to find which change exactly
trigerred a bug.  (Granted, this would make debugging version slow as
hell, but with this rate of error occurence it's not like you'd need
to run it for days.)

Without automated error detection tools cc-mode is bound to have
similar issues after every second non-trivial change.

Paul