Basic: make the nan family pure

The nan family of math routines do not rely on global state.  They do
however depend on their parameter.  This fits the description of pure:
  Functions which have no effects except the return value and their
  return value depends only on the parameters and/or global variables.
Mark the family as `readonly`.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315968 91177308-0d34-0410-b5e6-96231b3b80d8