[ubsan] Save the result of getLLVMContext. NFC.
[ubsan] Save the result of getLLVMContext. NFC.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@314751 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 9ce2dd4d7e6c33ff7f49556f5b70f246ea204756)

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