CodeGen: extract some local variables in CFConstantString creation (NFC)
CodeGen: extract some local variables in CFConstantString creation (NFC)

Extract the reference to the ASTContext and Triple and use them throughout the
function.  This is simply a cosmetic cleanup while in the area.  NFC.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@345160 91177308-0d34-0410-b5e6-96231b3b80d8

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