C++: Fix handling of std::va_list that is used as a function parameter

In the Unix ABI, `std::va_list` is defined as `typedef struct __va_list_tag { ... } va_list[1];`, which means that any `std::va_list` used as a function parameter decays to `struct __va_list_tag*`. Handling this actually made the QL code slightly cleaner. The only tricky bit is that we have to determine what type to use as the actual `va_list` type when loading, storing, or modifying a `std::va_list`. To do this, we look at the type of the argument to the `va_*` macro. A detailed QLDoc comment explains the details.

I added a test case for passing a `va_list` as an argument, and then manipulating that `va_list` in the callee.

cpp/ql/test/library-tests/ir/ir/ir.cpp

@@ -885,6 +885,14 @@ void FuncPtrConversions(int(*pfn)(int), void* p) {
   pfn = (int(*)(int))p;
 }
 
+void VAListUsage(int x, __builtin_va_list args) {
+  __builtin_va_list args2;
+  __builtin_va_copy(args2, args);
+  double d = __builtin_va_arg(args, double);
+  float f = __builtin_va_arg(args, int);
+  __builtin_va_end(args2);
+}
+
 void VarArgUsage(int x, ...) {
   __builtin_va_list args;
 
@@ -894,6 +902,7 @@ void VarArgUsage(int x, ...) {
   double d = __builtin_va_arg(args, double);
   float f = __builtin_va_arg(args, int);
   __builtin_va_end(args);
+  VAListUsage(x, args2);
   __builtin_va_end(args2);
 }