fix D78849 for g++ < 7.1

Summary:
Looks like g++ < 7.1 has a bug resolving calls to member functions without
`this->` in lamdas with `auto` types.  It looks like multiple build bots are
using g++-5.

https://stackoverflow.com/questions/32097759/calling-this-member-function-from-generic-lambda-clang-vs-gcc
https://godbolt.org/z/MiaRt-

Reviewers: MaskRay, efriedma, jyknight, craig.topper, rsmith

Reviewed By: rsmith

Subscribers: hiraditya, llvm-commits, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78962