[HLSL] Call global destructors from entries

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