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[X86][CET] Changing -fcf-protection behavior to comply with gcc (LLVM part)

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[X86][CET] Changing -fcf-protection behavior to comply with gcc (LLVM part)

This patch aims to match the changes introduced in gcc by
https://gcc.gnu.org/ml/gcc-cvs/2018-04/msg00534.html. The
IBT feature definition is removed, with the IBT instructions
being freely available on all X86 targets. The shadow stack
instructions are also being made freely available, and the
use of all these CET instructions is controlled by the module
flags derived from the -fcf-protection clang option. The hasSHSTK
option remains since clang uses it to determine availability of
shadow stack instruction intrinsics, but it is no longer directly used.

Comes with a clang patch (D46881).

Patch by mike.dvoretsky

Differential Revision: https://reviews.llvm.org/D46882

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