C++ exceptions in libc++ currently unwind through main instead of crashing with a backtrace

[danakj]

https://discord.com/channels/655572317891461132/655578254970716160/1350190049479884822

There is some mysterious flag we need to pass to clang, since our libc++ is compiled with exceptions enabled to make them abort when they reach the edge of exception-enabled code.

[danakj]

I don't actually see us passing -fno-exceptions or -fno-cxx-exceptions in our bazel rules, so not sure if the diagnosis above from discord is correct.

[danakj]

I also don't see any obvious flags in https://clang.llvm.org/docs/ClangCommandLineReference.html related to this.