Implement PR5242: don't desugar a type more than once in a diagnostic. This

implements a framework that allows us to use information about previously
substituted values to simplify subsequent ones.  Maybe this would be useful
for C++'y stuff, who knows.  We now get:

t.c:4:21: error: invalid operands to binary expression ('size_t' (aka 'unsigned long *') and 'size_t')
  return (size_t) 0 + (size_t) 0;
         ~~~~~~~~~~ ^ ~~~~~~~~~~

on the testcase.  Note that size_t is only aka'd once.

llvm-svn: 84604