[clang-tidy] Fix invalid fixit for readability-static-accessed-through-instance (bug 40544)

Summary:
Fixed https://bugs.llvm.org/show_bug.cgi?id=40544
Before, we would generate a fixit like `(anonymous namespace)::Foo::fun();` for
the added test case.

Reviewers: aaron.ballman, alexfh, xazax.hun

Subscribers: rnkovacs, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D61874

llvm-svn: 360698