[Stack Protection] Add diagnostic information for why stack protection was applied to a function

Stack Smash Protection is not completely free, so in hot code, the overhead it causes can cause performance issues. By adding diagnostic information for which functions have SSP and why, a user can quickly determine what they can do to stop SSP being applied to a specific hot function.

This change adds a remark that is reported by the stack protection code when an instruction or attribute is encountered that causes SSP to be applied.

Patch by: James Henderson

Differential Revision: https://reviews.llvm.org/D29023

llvm-svn: 296483