C++: Enable cpp/alloca-in-loop on LGTM

Now that the query has both tests and qhelp, we can use it on LGTM. This
commit also adds a change note.

I renamed the query to reduce confusion from the lower-case unquoted
word "alloca".

Author: jbj

change-notes/1.21/analysis-cpp.md

@@ -8,6 +8,7 @@
 |-----------------------------|-----------|--------------------------------------------------------------------|
 | `()`-declared function called with too few arguments (`cpp/too-few-arguments`) | Correctness | Find all cases where the number of arguments is less than the number of parameters of the function, provided the function is also properly declared/defined elsewhere. |
 | `()`-declared function called with mismatched arguments (`cpp/mismatched-function-arguments`) | Correctness | Find all cases where the types of arguments do not match the types of parameters of the function, provided the function is also properly declared/defined elsewhere. |
+| Call to alloca in a loop (`cpp/alloca-in-loop`) | reliability, correctness, external/cwe/cwe-770 | Finds calls to `alloca` in loops, which can lead to stack overflow if the number of iterations is large.  Newly displayed on LGTM. |
 
 ## Changes to existing queries
 

cpp/ql/src/Likely Bugs/Memory Management/AllocaInLoop.ql

@@ -1,11 +1,13 @@
 /**
- * @name alloca in a loop
+ * @name Call to alloca in a loop
  * @description Using alloca in a loop can lead to a stack overflow
  * @kind problem
  * @problem.severity warning
+ * @precision high
  * @id cpp/alloca-in-loop
  * @tags reliability
  *       correctness
+ *       security
  *       external/cwe/cwe-770
  */