Implement the core checking for compatible exception specifications in...

Implement the core checking for compatible exception specifications in assignment and initialization.

The exception specification of the assignee must be the same or a subset of the target. In addition, exception specifications on arguments and return types must be equivalent, but this is not implemented yet.
This currently produces two diagnostics for every invalid assignment/initialization, due to the diagnostic produced outside PerformImplicitConversion, e.g. in CheckSingleInitializer. I don't know how to suppress this; in any case I think it is the wrong place for a diagnostic, since there are other diagnostics produced inside the function. So I'm leaving it as it is for the moment.

llvm-svn: 83710