swift234

[ClangImporter] Define header guards for <iso646.h> so it's never present.

This is a hack to work around two issues:
- <iso646.h>, which defines macros for "and", "or", and "not" (among other
  things) is an implicit submodule of Darwin.
- Macros even in explicit submodules are leaking out when the parent module
  is imported <rdar://problem/14870036>.

There's no actual reason to require <iso646.h> in SDK header files -- it
should be a user-level choice whether or not to use those names. And
selectors with "and", "or", and "not" in them should not be mangled by this.

So, as a hack, we define the header guards that <iso646.h> uses ahead of
time, so that the file will be ignored. We do this for /both/ variants of
<iso646.h> on our system (Clang's and /usr/include's) just to be safe.

<rdar://problem/17110619>

Swift SVN r19822