Zcu: don't tell linkers about exports if there are compile errors

In the best case, this is redundant work, because we aren't actually
going to emit a working binary this update. In the worst case, it causes
bugs because the linker may not have *seen* the thing being exported due
to the compile errors.

Resolves: #24417

test/cases/compile_errors/exported_function_uses_invalid_type.zig

@@ -0,0 +1,13 @@
+export fn foo() void {
+    const S = struct { x: u32 = "bad default" };
+    const s: S = undefined;
+    _ = s;
+}
+
+// This test case explicitly runs on the LLVM backend as well as self-hosted, as
+// the original bug leading to this test occurred only with the LLVM backend.
+
+// error
+// backend=stage2,llvm
+//
+// :2:33: error: expected type 'u32', found '*const [11:0]u8'