[analyzer] Fixing SVal::getType returns Null Type for NonLoc::ConcreteInt in boolean type

In method `TypeRetrievingVisitor::VisitConcreteInt`, `ASTContext::getIntTypeForBitwidth` is used to get the type for `ConcreteInt`s.
However, the getter in ASTContext cannot handle the boolean type with the bit width of 1, which will make method `SVal::getType` return a Null `Type`.
In this patch, a check for this case is added to fix this problem by returning the bool type directly when the bit width is 1.

Differential Revision: https://reviews.llvm.org/D129737