[analyzer] Merge RegionStore's KillStruct and CopyLazyBindings: BindAggregate.

Both methods need to clear out existing bindings and provide a new default
binding. Originally KillStruct always provided UnknownVal as the default,
but it's allowed symbolic values for quite some time (for handling returned
structs in C).

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161637 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed