Basic: adjust attributes on `nan` LIBBUILTINs

The `nan` family of functions will inspect the contents of the parameter
that they are passed. As a result, the function cannot be annotated as
`const`.  The documentation of the `const` attribute explicitly states
this:
  Note that a function that has pointer arguments and examines the data
  pointed to must not be declared const.
Adjust the annotations on this family of functions.

llvm-svn: 315741