HLSL doesn't have a C++ runtime that supports `atexit` registration. To
enable global destructors we instead rely on the `llvm.global_dtor`
mechanism.
This change disables `atexit` generation for HLSL and updates the HLSL
code generation to call global destructors on the exit from entry
functions.
Depends on D132977.
Reviewed By: aaron.ballman
Differential Revision: https://reviews.llvm.org/D133518