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

Summary:
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

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54555

llvm-svn: 347586