Teach Sema::MergeFunctionDecl to properly check for an out-of-line definition...

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.

llvm-svn: 269935