Refactor template type diffing

1) Instead of using pairs of From/To* fields, combine fields into a struct
TemplateArgInfo and have two in each DiffNode.
2) Use default initialization in DiffNode so that the constructor shows the
only field that is initialized differently on construction.
3) Use Set and Get functions per each DiffKind to make sure all fields for the
diff is set.  In one case, the Expr fields were not set.
4) Don't print boolean literals for boolean template arguments.  This prevents
printing 'false aka 0'

Only #3 has a functional change, which is reflected in the test change.

llvm-svn: 257831