stage2: first pass at implementing usingnamespace

Ran into a design flaw here which will need to get solved by having
AstGen annotate ZIR with which instructions are closed over.

test/compile_errors.zig

@@ -8846,4 +8846,14 @@ pub fn addCases(ctx: *TestContext) !void {
     , &[_][]const u8{
         "error: invalid operands to binary expression: 'f32' and 'f32'",
     });
+
+    ctx.objErrStage1("undeclared identifier in unanalyzed branch",
+        \\export fn a() void {
+        \\    if (false) {
+        \\        lol_this_doesnt_exist = nonsense;
+        \\    }
+        \\}
+    , &[_][]const u8{
+        "tmp.zig:3:9: error: use of undeclared identifier 'lol_this_doesnt_exist'",
+    });
 }