Fixing more issues with template type diffing

1) Print qualifiers for templates with zero arguments
2) Add a few more tests for the template type diffing refactoring.
Specifically, PR24587 has been fixed and has a test case from
http://reviews.llvm.org/D15384
3) Adds asserts to check the DiffTree is in correct state when moving nodes
4) Rename the field FromType and ToType since it is heavily used within
member functions.

llvm-svn: 257870