[analyzer] Tidy up a few uses of Optional in RegionStore.
[analyzer] Tidy up a few uses of Optional in RegionStore.

Some that I just added needed conversion to use 'None', others looked
better using Optional<SVal>::create.

No functionality change.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@175714 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang