Now that macro expansion notes are real notes and go through the same

formatting as any other diagnostic, they will be properly line wrapped and
otherwise pretty printed. Let's take advantage of that and the new factoring to
add some helpful information to them (much like template backtrace notes and
other notes): the name of the macro whose expansion is being noted. This makes
a world of difference if caret diagnostics are disabled, making the expansion
notes actually useful in this case. It also helps ensure that in edge cases the
information the user needs is present. Consider:

% nl -ba t5.cc
     1  #define M(x, y, z) \
     2    y
     3
     4  M(
     5    1,
     6    2,
     7    3);

We now produce:
% ./bin/clang -fsyntax-only t5.cc
t5.cc:6:3: error: expected unqualified-id
  2,
  ^
t5.cc:2:3: note: expanded from macro: M
  y
  ^
1 error generated.

Without the added information in the note, the name of the macro being expanded
would never be shown.

This also deletes a FIXME to use the diagnostic formatting. It's not yet clear
to me that we *can* do this reasonably, and the production of this message was
my primary goal here anyways.

I'd love any comments or suggestions on improving these notes, their wording,
etc. Currently, I need to make them provide more helpful information in the
presence of a token-pasting buffer, and I'm pondering adding something along
the lines of "expanded from argument N of macro: ...".

llvm-svn: 142127