Fix test for the hasExternalFormalLinkage matcher

Summary:
Names of local variables have no linkage (see C++20 [basic.link] p8).

Names of variables in unnamed namespace have internal linkage (see C++20
[basic.link] p4).

Reviewers: aaron.ballman, rsmith, ymandel

Reviewed By: ymandel

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83700