Update the %diff modifer to have an alternate string print when a template tree

Update the %diff modifer to have an alternate string print when a template tree
is selected.  This will allow more flexibility when converting diagnostics to
use template type diffing.

Also updated the internal manual and test cases for correctly keeping the bold
attribute and for tree printing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159463 91177308-0d34-0410-b5e6-96231b3b80d8