Add a couple more headers. stddef was discussed on cfe-dev, and

consensus was that it was fine; iso646 is trivial.

We actually don't need that many headers overall... on Ubuntu, at least, 
all we need are stdint.h, stdarg.h, limits.h, and float.h to stop 
depending on having gcc's headers around for normal code.

On a side note, the path searching needs some improvement; I had to 
hardcode the path to my header directory into clang.cpp to get the 
driver to use it consistently.

llvm-svn: 51317