[mlir] Allow accessing DialectResourceBlobManager::blobMap (cherry-pick) (llvm#142)

Add a new API to access all blobs that are stored in the blob manager.
The main purpose (as of now) is to allow users of dialect resources to
iterate over all blobs, especially when the blobs are no longer used in
IR (e.g. the operation that uses the blob is deleted) and thus cannot be
easily accessed without manual tracking of keys.

Author: andrey-golubev

mlir/unittests/IR/CMakeLists.txt

@@ -19,7 +19,6 @@ add_mlir_unittest(MLIRIRTests
   TypeTest.cpp
   TypeAttrNamesTest.cpp
   OpPropertiesTest.cpp
-  ValueTest.cpp
   BlobManagerTest.cpp
 
   DEPENDS