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