Add concurrent caller location test

tests/run-pass/concurrency/concurrent_caller_location.rs

@@ -0,0 +1,17 @@
+use std::thread::spawn;
+use std::panic::Location;
+
+fn initialize() {
+    let _ignore = initialize_inner();
+}
+
+fn initialize_inner() -> &'static Location<'static> {
+    Location::caller()
+}
+
+fn main() {
+    let j1 = spawn(initialize);
+    let j2 = spawn(initialize);
+    j1.join().unwrap();
+    j2.join().unwrap();
+}

tests/run-pass/concurrency/concurrent_caller_location.stderr

@@ -0,0 +1,2 @@
+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
+