C++1y deduced return types: when we deduce a return type for a function which
we loaded from PCH, if we're building another PCH, create an update record to
patch the return type of the earlier declaration.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181659 91177308-0d34-0410-b5e6-96231b3b80d8