add a prototype for __builtin_fpclassify. We don't know how to

codegen it, so we get 'cannot compile this builtin function yet'
errors, just like we do currently for __builtin_isinf.  However,
this should let us parse headers that use it without barfing,
which should help PR6083.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@94346 91177308-0d34-0410-b5e6-96231b3b80d8