AST: Avoid reading an unitialized value
AST: Avoid reading an unitialized value

Desugar doesn't necessarily initialize ShouldAKA, but as of r241542 it
may read it. Fix the misuse of the API and initialize this before
passing it in.

Found by ubsan.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@241705 91177308-0d34-0410-b5e6-96231b3b80d8

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