[clang-tidy] minor bug fix to AbseilMatcher.h

This missing directory is not yet released, but is causing some problems
internally. It's gonna be released eventually and received permission to
include it here. This matcher will also be periodically updated by my
team as we have more releases and or problems internally.

Patch by Hugo Gonzalez!

Differential Revision: https://reviews.llvm.org/D51699

llvm-svn: 341488