Declaration::isResult now always returns 0

(storage_class & STCxxx) may overflow if STCxxx is bigger than int.max
fix Issue 9413 - Incorrect modification inside contracts is not detected correctly
fix Issue 9414 - Incorrect modification inside contracts is not detected on virtual function
Fix up error messages for 9414
Merge pull request #1569 from 9rnsr/fix_contracts

Issue 9413 & 9414 - Detect incorrect modification inside contracts