[stackprotector] Update the StackProtector pass to perform datalayout analysis.

This modifies the pass to classify every SSP-triggering AllocaInst according to
an SSPLayoutKind (LargeArray, SmallArray, AddrOf).  This analysis is collected
by the pass and made available for use, but no other pass uses it yet.

The next patch will make use of this analysis in PEI and StackSlot
passes.  The end goal is to support ssp-strong stack layout rules.

WIP.

Differential Revision: http://llvm-reviews.chandlerc.com/D1789

llvm-svn: 193653