reimplement __sync_* builtins to be variadic and to follow the same

reimplement __sync_* builtins to be variadic and to follow the same
semantic rules that gcc and icc use.  This implements the variadic
and concrete versions as builtins and has sema do the
disambiguation.  There are probably a bunch of details to finish up
but this seems like a large monotonic step forward :)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@71212 91177308-0d34-0410-b5e6-96231b3b80d8