Teach Sema::MergeFunctionDecl to properly check for an out-of-line definition of a function that is declared as =default in its class definition.

First part of PR27699.

Patch by Cristina Cristescu!

Reviewed by Richard Smith and me.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@269935 91177308-0d34-0410-b5e6-96231b3b80d8