[analyzer] Unroll the loop when it has a unsigned counter.

Summary:
The original implementation in the `LoopUnrolling.cpp` didn't consider the case where the counter is unsigned. This case is only handled in `simpleCondition()`, but this is not enough, we also need to deal with the unsinged counter with the counter initialization.

Since `IntegerLiteral` is `signed`, there is a `ImplicitCastExpr<IntegralCast>` in `unsigned counter = IntergerLiteral`. This patch add the `ignoringParenImpCasts()` in the `IntegerLiteral` matcher.

Reviewers: szepet, a.sidorin, NoQ, george.karpenkov

Reviewed By: szepet, george.karpenkov

Subscribers: xazax.hun, rnkovacs, cfe-commits, MTC

Differential Revision: https://reviews.llvm.org/D45086

llvm-svn: 328919