[C++] Support `__builtin_observable_checkpoint`

[cor3ntin]

To support P1494 we need a built-in that lowers to an IR instruction that would behave like an opaque function and would resist time travel UB during optimisation.

__builtin_observable_checkpoint is the name that has been chosen by GCC.

I'm happy to implement the builtin in clang once we know what to lower it too.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Corentin Jabot (cor3ntin)

To support [P1494](wg21.link/P1494) we need a built-in that lowers to an IR instruction that would behave like an opaque function and would resist time travel UB during optimisation.

__builtin_observable_checkpoint is the name that has been chosen by GCC.

I'm happy to implement the builtin in clang once we know what to lower it too.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Corentin Jabot (cor3ntin)

To support [P1494](wg21.link/P1494) we need a built-in that lowers to an IR instruction that would behave like an opaque function and would resist time travel UB during optimisation.

__builtin_observable_checkpoint is the name that has been chosen by GCC.

I'm happy to implement the builtin in clang once we know what to lower it too.