[analyzer] Evaluate integral casts as cast symbols if truncations are detected.

The current workaround for truncations not being modelled is that the evaluation of integer to integer casts are simply bypassed and so the original symbol is used as the new casted symbol (cf SimpleSValBuilder::evalCastFromNonLoc).
This lead to the issue described in PR25078, as the RangeConstraintManager associates ranges with symbols.

The new evalIntegralCast method added by this patch wont bypass the cast if it finds the range of the symbol to be greater than the maximum value of the target type.

The fix to RangeSet::pin mentioned in the initial review will be committed separately.

Differential Revision: http://reviews.llvm.org/D12901

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@257464 91177308-0d34-0410-b5e6-96231b3b80d8