Various builtins a require an integer constant. Not providing

one results in an obscure error from the backend that doesn't make
sense.  Capture this information in the .def files for builtins with
a new "I" modifier letter indicating the an argument needs to be an ICE.

Nothing uses this yet, but sema will eventually enforce this (eliminating
some special cases from semachecking.cpp) and codegen will use this to 
force an ICE value, preventing backend error messages.

llvm-svn: 115302