[Clang] Implement function attribute no_stack_protector.

Summary:
This attribute tells clang to skip this function from stack protector
when -stack-protector option is passed.
GCC option for this is:
__attribute__((__optimize__("no-stack-protector"))) and the
equivalent clang syntax would be: __attribute__((no_stack_protector))

This is used in Linux kernel to selectively disable stack protector
in certain functions.

Reviewers: aaron.ballman, rsmith, rnk, probinson

Reviewed By: aaron.ballman

Subscribers: probinson, srhines, cfe-commits

Differential Revision: https://reviews.llvm.org/D46300

llvm-svn: 331925