Driver: Add -[no-]integrated-as for clang.

 - Requires backend support, which only exists for i386--darwin currently.

No 'as' required:
--
ddunbar@ozzy:tmp$ cat t.c
int main() { return 42; }
ddunbar@ozzy:tmp$ clang -m32 -integrated-as t.c


ddunbar@ozzy:tmp$ ./a.out; echo $?
42
ddunbar@ozzy:tmp$
--

The random extra whitespace is how you know its working! :)

llvm-svn: 95194