[CAS] Fix msan error in ObjectStore::importObject()

[cachemeifyoucan]

Fix msan error that reference to ObjectRef is used after the storage is
deleted. Make sure all usages is finished before deleting the container.

[llvm-ci]

LLVM Buildbot has detected a new failure on builder ppc64le-mlir-rhel-clang running on ppc64le-mlir-rhel-test while building llvm at step 3 "clean-build-dir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/129/builds/32703

Here is the relevant piece of the build log for the reference

Step 3 (clean-build-dir) failure: Delete failed. (failure) (timed out)
Step 5 (build-check-mlir-build-only) failure: build (failure) (timed out)
...
1072.799 [59/243/4183] Linking CXX static library lib/libMLIRTestToLLVMIRTranslation.a
1072.809 [59/242/4184] Linking CXX static library lib/libMLIRTestIRDLToCppDialect.a
1072.810 [59/241/4185] Linking CXX static library lib/libMLIRFuncTestPasses.a
1072.845 [59/240/4186] Linking CXX static library lib/libMLIRTestIR.a
1072.870 [59/239/4187] Linking CXX static library lib/libMLIRTestPDLL.a
1073.611 [59/238/4188] Linking CXX executable tools/mlir/unittests/Dialect/ArmSME/MLIRArmSMETests
1074.475 [59/237/4189] Linking CXX executable tools/mlir/unittests/IR/MLIRIRTests
1074.655 [59/236/4190] Linking CXX executable tools/mlir/unittests/TableGen/MLIRTableGenTests
1074.847 [59/235/4191] Linking CXX executable tools/mlir/unittests/Parser/MLIRParserTests
1079.226 [59/234/4192] Linking CXX executable tools/mlir/unittests/Transforms/MLIRTransformsTests
command timed out: 1200 seconds without output running [b'ninja', b'check-mlir-build-only'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2280.208386