[analyzer][NFC] Add xrefs to a test case that has poor git blame (#145501)

Author: balazs-benics-sonarsource

clang/test/Analysis/PR38208.c

@@ -2,6 +2,11 @@
 
 // expected-no-diagnostics
 
+// This test case used to demonstrate a huge slowdown regression.
+// Reported in https://bugs.llvm.org/show_bug.cgi?id=38208
+// Caused by 2bbccca9f75b6bce08d77cf19abfb206d0c3bc2e aka. "aggressive-binary-operation-simplification"
+// Fixed by dcde8acc32f1355f37d3bc2814c528fdc2ca5f94
+
 int foo(int a, int b) {
   a += b; b -= a;
   a += b; b -= a;