[X86] Implement -fzero-call-used-regs option

The "-fzero-call-used-regs" option tells the compiler to zero out
certain registers before the function returns. It's also available as a
function attribute: zero_call_used_regs.

The two upper categories are:

  - "used": Zero out used registers.
  - "all": Zero out all registers, whether used or not.

The individual options are:

  - "skip": Don't zero out any registers. This is the default.
  - "used": Zero out all used registers.
  - "used-arg": Zero out used registers that are used for arguments.
  - "used-gpr": Zero out used registers that are GPRs.
  - "used-gpr-arg": Zero out used GPRs that are used as arguments.
  - "all": Zero out all registers.
  - "all-arg": Zero out all registers used for arguments.
  - "all-gpr": Zero out all GPRs.
  - "all-gpr-arg": Zero out all GPRs used for arguments.

This is used to help mitigate Return-Oriented Programming exploits.

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D110869

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