[ADT] Try again to use the same version of llvm::Optional on all compilers
[ADT] Try again to use the same version of llvm::Optional on all compilers

The miscompile doesn't reproduce for me anymore with GCC 7.3. I'll watch
the buildbots closely.

Having different versions of Optional is an ABI violation when linking
GCC- and clang-built code together.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@342637 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm