Reduce conversions between Store <-> ImmutableMapRef in RegionStore.
Reduce conversions between Store <-> ImmutableMapRef in RegionStore.

This reduces canonicalization of ImmutableMaps.  This reduces analysis time
of one heavy Objective-C file by another 1%.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@169630 91177308-0d34-0410-b5e6-96231b3b80d8

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