C++: Upgrade query precision.

Author: geoffw0

cpp/ql/src/Likely Bugs/Underspecified Functions/ImplicitFunctionDeclaration.ql

@@ -5,7 +5,7 @@
  * may lead to unpredictable behavior.
  * @kind problem
  * @problem.severity warning
- * @precision medium
+ * @precision high
  * @id cpp/implicit-function-declaration
  * @tags correctness
  *       maintainability