Revert "[clang-tidy] Handle unresolved expressions in ExprMutationAnalyzer"

Summary:
Tests somehow break on windows (and only on windows)
http://lab.llvm.org:8011/builders/clang-x64-ninja-win7/builds/13003
http://lab.llvm.org:8011/builders/clang-x86-windows-msvc2015/builds/13747

I have yet figure out why so reverting to unbreak first.

Reviewers: george.karpenkov

Subscribers: xazax.hun, a.sidorin, Szelethus, cfe-commits

Differential Revision: https://reviews.llvm.org/D51898

llvm-svn: 341886