[RFC] Constant-Time Coding Support

Constant-Time Coding Support Summary We (@kumarak, @frabert, @hbrodin, @wizardengineer, and myself of Trail of Bits) propose a Clang “constant-time selection” builtin for cryptographers to use to ensure that their compiled C and C++ code selects between values in constant time consistently across target architectures. Our builtin will selectively bypass optimizations that are beneficial for most compiled code but that can replace intended constant-time operations with variable-time jumps or bran...