[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


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201120 91177308-0d34-0410-b5e6-96231b3b80d8