[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 function 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 an SSP-specific DiagnosticInfo class and uses of it to the Stack Protection code. A subsequent change to clang will cause the remarks to be emitted when enabled.

Patch by: James Henderson

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

llvm-svn: 294590