Fix PR27601 by reverting [r267453] - Refactor traversal of bases in deduction...

Fix PR27601 by reverting [r267453] - Refactor traversal of bases in deduction of template parameters from base

This reversal is being done with r267453's author's (i.e. Richard Smith's) permission.

This fixes https://llvm.org/bugs/show_bug.cgi?id=27601 

Also, per Richard's request the examples from the bug report have been added to our test suite.

llvm-svn: 270016