[llvm][cas] Fix stack-use-after-scope OnDiskCASLoggerTest

We were unintentionally capturing the loop induction variable I as a
reference instead of a copy. Caught by asan.

(cherry picked from commit ce6efb2)

rdar://147936276

Author: benlangmuir

llvm/unittests/CAS/OnDiskCASLoggerTest.cpp

@@ -106,7 +106,7 @@ TEST(OnDiskCASLoggerTest, MultiThread) {
                     Succeeded());
 
   for (int I = 0; I < 10; ++I) {
-    Pool.async([&] {
+    Pool.async([I, &SharedLogger, &Dir] {
       std::unique_ptr<OnDiskCASLogger> OwnedLogger;
       OnDiskCASLogger *Logger;
       // Mix using a shared instance and opening new instances in the same log.