[cfe-dev] PCH serialization issues (PR 34728)

[Steve O'Brien via cfe-dev]

I'm running into trouble when serializing a template function
declaration to PCH and subsequently reading it back:
https://bugs.llvm.org/show_bug.cgi?id=34728

Small example:

  header.h
  --
  template <int i=42, class T> int f(T const&);
  template <int i, class T> int f(T const&) {return i;}

  main.cpp
  --
  int main(int, char**) {return f(3.14);}

An error is reported that the value for `i` could not be deduced,
because in the AST -> PCH -> AST conversions the default value `42` is
lost.

I am just now finishing up a patch which adds one more item to the
serialized output for these template types, indicating the decl (with
`Record.AddDeclRef`) inherited from if present.

This seems to be working: fixes the errors seen in my repro (see
bugzilla link above for the repro patch), and the changes seem mostly
clean.  :)

Does this approach sound ok in theory?

Thanks!

--steveo