correct caching and add test for missing fn name

test/compile_errors.zig

@@ -6,6 +6,13 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\export async fn foo() void {}
     , "tmp.zig:1:1: error: exported function cannot be async");
 
+    cases.addExe(
+        "main missing name",
+        \\pub fn (main) void {}
+    ,
+        "tmp.zig:1:5: error: missing function name",
+    );
+
     cases.addCase(x: {
         var tc = cases.create("@newStackCall on unsupported target",
             \\export fn entry() void {