Revert r342637 "[ADT] Try again to use the same version of llvm::Optional on all compilers"

and also revert follow-ups r342643 and r342723.

This caused Clang to be miscompiled by GCC 4.8.4 (Unbuntu 14.04's
default compiler) and break the Chromium build (see
https://crbug.com/888061).

llvm-svn: 342966