Similar to clang 3.3, it seems that clang 3.4 declares an incompatible
function prototypes for __builtin_isnan(). To workaround this issue,
we should not declare the function prototype for __builtin_isnan().
Note: Though it may be good to fix this in clang type checker, like
__builtin_nexttoward(), however it seems non-trivial since __builtin_isnsn()
might be overloaded. We can revert this after we have fixed the
clang builtin definition.
Change-Id: I329b56225f3d229b0c141bb8414c4369f154efda
ad40d952a1