Control-Flow Enforcement Technology - Shadow Stack and Indirect Branch Tracking suppo...

Control-Flow Enforcement Technology - Shadow Stack and Indirect Branch Tracking support (Clang side)

Shadow stack solution introduces a new stack for return addresses only.
The stack has a Shadow Stack Pointer (SSP) that points to the last address to which we expect to return.
If we return to a different address an exception is triggered.
This patch includes shadow stack intrinsics as well as the corresponding CET header.
It includes CET clang flags for shadow stack and Indirect Branch Tracking.

For more information, please see the following:
https://software.intel.com/sites/default/files/managed/4d/2a/control-flow-enforcement-technology-preview.pdf

Differential Revision: https://reviews.llvm.org/D40224

Change-Id: I79ad0925a028bbc94c8ecad75f6daa2f214171f1
llvm-svn: 318995