[clang][slh] add attribute for speculative load hardening
The prior diff had to be reverted because there were two tests
that failed. I updated the two tests in this diff
- Summary from Previous Diff (Still Accurate) -----
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
Differential Revision: https://reviews.llvm.org/D54915