Prevent analysis of functions only referenced at comptime

The idea here is that there are two ways we can reference a function at runtime:

* Through a direct call, i.e. where the function is comptime-known
* Through a function pointer

This means we can easily perform a form of rudimentary escape analysis
on functions. If we ever see a `decl_ref` or `ref` of a function, we
have a function pointer, which could "leak" into runtime code, so we
emit the function; but for a plain `decl_val`, there's no need to.

This change means that `comptime { _ = f; }` no longer forces a function
to be emitted, which was used for some things (mainly tests). These use
sites have been replaced with `_ = &f;`, which still triggers analysis
of the function body, since you're taking a pointer to the function.

Resolves: #6256
Resolves: #15353

lib/std/multi_array_list.zig

@@ -532,8 +532,8 @@ pub fn MultiArrayList(comptime T: type) type {
 
         comptime {
             if (builtin.mode == .Debug) {
-                _ = dbHelper;
-                _ = Slice.dbHelper;
+                _ = &dbHelper;
+                _ = &Slice.dbHelper;
             }
         }
     };