As an experimental hack, emit "instantiated from" information in

diagnostics.  I'm not sure I want to keep this, but hey, it's easy
and could be useful or something, even if guarded by a 
-fshow-me-tons-of-details option.  A silly example is:

#define A B
#define C A
#define D C

int y = D;

We now emit:

t.c:11:9: error: use of undeclared identifier 'B'
int y = D;
        ^
t.c:9:11: note: instantiated from:
#define D C
          ^
t.c:8:11: note: instantiated from:
#define C A
          ^
t.c:7:11: note: instantiated from:
#define A B
          ^

A more useful example is from tgmath:

t.c:4:9: error: no matching function for call to '__tg_acos'
 return acos(x);
        ^~~~~~~
/Users/sabre/llvm/Debug/Headers/tgmath-sofar.h:51:17: note: instantiated from:
#define acos(x) __tg_acos(x)
                ^
... candidate set follows ...

This does not yet print ranges in instantiation info, (e.g. highlighting the
range "__tg_acos(x)" in the last example), but that could be added if we 
decide this is a good idea :).

Thoughts and bug reports welcome!

llvm-svn: 64761