Fix processing of -Ufoo to not inject "#undef foo 1" into the predefines

buffer.  This caused exciting nonsense like this:

$ clang t.c -fsyntax-only -UMACRO
In file included from <built-in>:104:
<command line>:1:14: warning: extra tokens at end of #undef directive [-Wextra-tokens]
#undef MACRO 1
             ^
             //
1 diagnostic generated.

rdar://6891800

llvm-svn: 71860