[OpenCL] Eliminate warning when declaring OpenCL builtin functions.

OpenCL builtin functions are usually declared in header files.
Currently clang emits warning for OpenCL builtin functions
which have the same name as standard C library functions.

This commit eliminates such warnings by not adding the C standard
includes following the restriction from OpenCL v1.2 s6.9.f.

Patch by Liu Yaxun (Sam)!

Review: http://reviews.llvm.org/D16692
llvm-svn: 259491