Revert "[clang][slh] add attribute for speculative load hardening"

until I figure out why the build is failing or timing out

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Summary:
The prior diff had to be reverted because there were two tests
that failed. I updated the two tests in this diff

clang/test/Misc/pragma-attribute-supported-attributes-list.test
clang/test/SemaCXX/attr-speculative-load-hardening.cpp

LLVM IR already has an attribute for speculative_load_hardening. Before
this commit, when a user passed the -mspeculative-load-hardening flag to
Clang, every function would have this attribute added to it. This Clang
attribute will allow users to opt into SLH on a function by function
basis.

This can be applied to functions and Objective C methods.

Reviewers: chandlerc, echristo, kristof.beyls, aaron.ballman

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54915

This reverts commit a5b3c232d1e3613f23efbc3960f8e23ea70f2a79.
(r347617)

llvm-svn: 347628