Add support for llvm.is.constant intrinsic (PR4898)

This adds the llvm-side support for post-inlining evaluation of the
__builtin_constant_p GCC intrinsic.

Also fixed SCCPSolver::visitCallSite to not blow up when seeing a call
to a function where canConstantFoldTo returns true, and one of the
arguments is a struct.

Updated from patch initially by Janusz Sobczak.

Differential Revision: https://reviews.llvm.org/D4276

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346322 91177308-0d34-0410-b5e6-96231b3b80d8