Suppress macro expansion of NULL in NULL warnings.

For "int i = NULL;" we would produce:

null.cpp:5:11: warning: implicit conversion of NULL constant to integer [-Wconversion]
  int i = NULL;
      ~   ^~~~
null.cpp:1:14: note: expanded from macro 'NULL'
\#define NULL __null
              ^~~~~~

But we really shouldn't trace that macro expansion back into the header, yet we
still want macro back traces for code like this:

\#define FOO NULL
int i = FOO;

or

\#define FOO int i = NULL;
FOO

While providing appropriate tagging at different levels of the expansion, etc.

The included test case exercises these cases & does some basic validation (to
ensure we don't have macro expansion notes where we shouldn't, and do where we
should) - but doesn't go as far as to validate the source location/ranges
used in those notes and warnings.

llvm-svn: 152940