Disable -Wformat-extra-args for arguments defined in system headers.

Some of the NSAssert macros in OS X 10.7 are implemented in a way that
adds extra arguments that trigger the -Wformat-extra-args warning.
Earlier versions of clang failed to detect those -Wformat issues, but now
that clang is reporting those problems, we need to quiet them since there's
nothing to be done to fix them.  <rdar://problem/11317765>

I don't know how to write a testcase for this.  Suggestions welcome.
Patch by Ted Kremenek!

llvm-svn: 156092