C++: Downgrade two queries to recommendation

The `cpp/local-variable-hides-global-variable` doesn't seem right as a
warning without some additional context. For example, is the local
variable and the global variable used in the same function body, and
do they have similar enough types that it would be possible to confuse
them.

The `cpp/missing-header-guard` query enforces good style and helps with
compilation speed, but AFAIK it has never flagged a correctness issue.
Therefore I think it should be a recommendation.

cpp/change-notes/2020-11-27-downgrade-to-recommendation.md

@@ -0,0 +1,2 @@
+lgtm,codescanning
+* The queries `cpp/local-variable-hides-global-variable` and `cpp/missing-header-guard` now have severity `recommendation` instead of `warning`.

cpp/ql/src/Best Practices/Hiding/LocalVariableHidesGlobalVariable.ql

@@ -2,7 +2,7 @@
  * @name Local variable hides global variable
  * @description A local variable or parameter that hides a global variable of the same name. This may be confusing. Consider renaming one of the variables.
  * @kind problem
- * @problem.severity warning
+ * @problem.severity recommendation
  * @precision very-high
  * @id cpp/local-variable-hides-global-variable
  * @tags maintainability

cpp/ql/src/jsf/4.07 Header Files/AV Rule 35.ql

@@ -4,7 +4,7 @@
  *              the file from being included twice). This prevents errors and
  *              inefficiencies caused by repeated inclusion.
  * @kind problem
- * @problem.severity warning
+ * @problem.severity recommendation
  * @precision high
  * @id cpp/missing-header-guard
  * @tags efficiency