[stackprotector] Add command line option -fstack-protector-strong

This option has the following effects:
 * It adds the sspstrong IR attribute to each function within the CU.
 * It defines the macro __SSP_STRONG__ with the value of 2.

Differential Revision: http://llvm-reviews.chandlerc.com/D2717

llvm-svn: 201120
9 files changed