[clang][dataflow] Merge `RecordValue`s with different locations correctly. (#65319)

Now that prvalue expressions map directly to values (see
https://reviews.llvm.org/D158977), it's no longer guaranteed that
`RecordValue`s
associated with the same expression will always have the same storage
location.

In other words, D158977 invalidated the assertion in
`mergeDistinctValues()`.
The newly added test causes this assertion to fail without the other
changes in
the patch.

This patch fixes the issue. However, the real fix will be to eliminate
the
`StorageLocation` from `RecordValue` entirely.