When instantiating the definition of a member function of a class

When instantiating the definition of a member function of a class
template, introduce that member function into the template
instantiation stack. Also, add diagnostics showing the member function
within the instantiation stack and clean up the qualified-name
printing so that we get something like:

  note: in instantiation of member function 'Switch1<int, 2, 2>::f'
  requested here

in the template instantiation backtrace.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@72015 91177308-0d34-0410-b5e6-96231b3b80d8