Fix for #972 when a static is used by a stock Function

This adds stock functions to the maybe used set which seems most appropriate.

We can then mark all of the functions they refer too as maybe used also.

compiler/semantics.cpp

@@ -2893,6 +2893,10 @@ bool Semantics::CheckFunctionDeclImpl(FunctionDecl* info) {
     if (info->as<MemberFunctionDecl>())
         maybe_used_.emplace_back(info);
 
+    // We never warn about unused stock functions.
+    if (info->is_stock())
+        maybe_used_.emplace_back(info);
+
     auto fwd = info->prototype();
     if (fwd && fwd->deprecate() && !info->is_stock())
         report(info->pos(), 234) << info->name() << fwd->deprecate();

tests/compile-only/ok-static-used-by-stock.inc

@@ -0,0 +1,7 @@
+static void SomeHelperFunction()
+{}
+
+stock void HelpMe()
+{
+	SomeHelperFunction();
+}

tests/compile-only/ok-static-used-by-stock.sp

@@ -0,0 +1,5 @@
+// warnings_are_errors: true
+#include "ok-static-used-by-stock.inc"
+
+public void main()
+{}