CGLoopInfo: Use the MD_loop metadata kind from r264371, NFC
CGLoopInfo: Use the MD_loop metadata kind from r264371, NFC

Besides a small compile-time speedup, there should be no real
functionality change here.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@264372 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 2250bd96d2b6fd52da1f14e6d94c5ecd1eaedf04)

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang