[analyzer] Do not conjure a symbol when we need to propagate taint.

When the solver and SValBuilder cannot reason about symbolic expressions (ex: (x+1)*y ), the analyzer conjures a new symbol with no ties to the past. This helps it to recover some path-sensitivity. However, this breaks the taint propagation.

With this commit, we are going to construct the expression even if we cannot reason about it later on if an operand is tainted.

Also added some comments and asserts.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@144932 91177308-0d34-0410-b5e6-96231b3b80d8