C++ IR: Fix SignAnalysis after getAnyDef -> getDef

In the `SignAnalysis` abstract interpretation, "unknown sign"
corresponds to the set of _all_ `Sign`, but using `getDef` leads to the
operand having _no_ `Sign`. To fix that, we assign all signs to inexact
operands.

cpp/ql/src/semmle/code/cpp/rangeanalysis/SignAnalysis.qll

@@ -368,6 +368,9 @@ cached module SignAnalysisCached {
     or
     result = guardedOperandSign(operand) and
     result = guardedOperandSignOk(operand)
+    or
+    // `result` is unconstrained if the definition is inexact. Then any sign is possible.
+    operand.isDefinitionInexact()
   }
   
   cached