[libc++] Avoid ODR violations in __exception_guard

Having an ODR violation with `__exception_guard` seems to be problematic in LTO builds. To avoid the ODR violation, give the class different names for exception/no-exceptions mode and have an alias to the correct class.

Reviewed By: ldionne, #libc, alexfh

Spies: aeubanks, dblaikie, joanahalili, alexfh, rupprecht, libcxx-commits

Differential Revision: https://reviews.llvm.org/D143071