C backend: fix enough that zig test works

* test_functions: properly add dependencies of the array on test
   functions and test names so that the order comes out correctly.
 * fix lowering of struct literals to add parentheses around the type
   name.
 * omit const qualifier in slices because otherwise slices cannot be
   reassigned even when they are local variables.
 * special case pointer to functions and double pointer to functions in
   renderTypeAndName. This code will need to be cleaned up but for now
   it helps us make progress on other C backend stuff.
 * fix slice element access to lower to `.ptr[` instead of `[`.
 * airSliceElemVal: respect volatile slices

src/Module.zig

@@ -4734,6 +4734,10 @@ pub fn populateTestFunctions(mod: *Module) !void {
             }),
             .val = try Value.Tag.array.create(arena, test_fn_vals),
         });
+
+        // Add a dependency on each test name and function pointer.
+        try array_decl.dependencies.ensureUnusedCapacity(gpa, test_fn_vals.len * 2);
+
         for (mod.test_functions.keys()) |test_decl, i| {
             const test_name_slice = mem.sliceTo(test_decl.name, 0);
             const test_name_decl = n: {
@@ -4747,6 +4751,8 @@ pub fn populateTestFunctions(mod: *Module) !void {
                 try test_name_decl.finalizeNewArena(&name_decl_arena);
                 break :n test_name_decl;
             };
+            array_decl.dependencies.putAssumeCapacityNoClobber(test_decl, {});
+            array_decl.dependencies.putAssumeCapacityNoClobber(test_name_decl, {});
             try mod.linkerUpdateDecl(test_name_decl);
 
             const field_vals = try arena.create([3]Value);