core: ensure callsites in test have unique addresses (#2681)

The test relies on TEST_CALLSITE_1 and TEST_CALLSITE_2 to have
different addresses. However, as they are zero-sized types, this
is not guaranteed.

This fixes the test failure with LLVM 17 and certain optimization
options reported at rust-lang/rust#114699.

tracing-core/src/field.rs

@@ -978,8 +978,9 @@ mod test {
     use super::*;
     use crate::metadata::{Kind, Level, Metadata};
 
-    struct TestCallsite1;
-    static TEST_CALLSITE_1: TestCallsite1 = TestCallsite1;
+    // Make sure TEST_CALLSITE_* have non-zero size, so they can't be located at the same address.
+    struct TestCallsite1(u8);
+    static TEST_CALLSITE_1: TestCallsite1 = TestCallsite1(0);
     static TEST_META_1: Metadata<'static> = metadata! {
         name: "field_test1",
         target: module_path!(),
@@ -999,8 +1000,8 @@ mod test {
         }
     }
 
-    struct TestCallsite2;
-    static TEST_CALLSITE_2: TestCallsite2 = TestCallsite2;
+    struct TestCallsite2(u8);
+    static TEST_CALLSITE_2: TestCallsite2 = TestCallsite2(0);
     static TEST_META_2: Metadata<'static> = metadata! {
         name: "field_test2",
         target: module_path!(),