[analyzer] Construct a SymExpr even when the constraint solver cannot

reason about the expression.

This essentially keeps more history about how symbolic values were
constructed. As an optimization, previous to this commit, we only kept
the history if one of the symbols was tainted, but it's valuable keep
the history around for other purposes as well: it allows us to avoid
constructing conjured symbols.

Specifically, we need to identify the value of ptr as
ElementRegion (result of pointer arithmetic) in the following code.
However, before this commit '(2-x)' evaluated to Unknown value, and as
the result, 'p + (2-x)' evaluated to Unknown value as well.

int *p = malloc(sizeof(int));
ptr = p + (2-x);

This change brings 2% slowdown on sqlite. Fixes radar://11329382.

llvm-svn: 155944