Fix a long standard problem with clang retaining "too much" sugar

information about types.  We often print diagnostics where we say 
"foo_t" is bad, but the user doesn't know how foo_t is declared 
(because it is a typedef).  Fix this by expanding sugar when present
in a diagnostic (and not one of a few special cases, like vectors).

Before:
t.m:5:2: error: invalid operands to binary expression ('typeof(P)' and 'typeof(F)')
 MAX(P, F);
 ^~~~~~~~~
t.m:1:78: note: instantiated from:
#define MAX(A,B)    ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a < __b ? __b : __a; })
                                                                             ^

After:
t.m:5:2: error: invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float'))
 MAX(P, F);
 ^~~~~~~~~
t.m:1:78: note: instantiated from:
#define MAX(A,B)    ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a < __b ? __b : __a; })
                                                                             ^

llvm-svn: 65081