C++: Precise printing of integer bounds

The pretty-printing of a QL `float` didn't include enough digits to tell
whether a large number had accurate bounds. The `toString` value of a
float appears to be more precise.

cpp/ql/test/library-tests/rangeanalysis/SimpleRangeAnalysis/lowerBound.ql

@@ -1,4 +1,4 @@
 import semmle.code.cpp.rangeanalysis.SimpleRangeAnalysis
 
 from VariableAccess expr
-select expr, lowerBound(expr)
+select expr, lowerBound(expr).toString()

cpp/ql/test/library-tests/rangeanalysis/SimpleRangeAnalysis/upperBound.ql

@@ -1,4 +1,4 @@
 import semmle.code.cpp.rangeanalysis.SimpleRangeAnalysis
 
 from VariableAccess expr
-select expr, upperBound(expr)
+select expr, upperBound(expr).toString()