[clang] add tests to ExprMutAnalyzer that reproduced a crash in ASTMatchers

Summary:
This patch adds two unit-tests that are the result of reducing a crashing TU
when running ExprMutAnalyzer over it. They are added only to ensure the regression
that has been fixed with https://reviews.llvm.org/D56444 don't creep back.

Reviewers: aaron.ballman, sammccall, rsmith, george.karpenkov

Reviewed By: sammccall

Subscribers: baloghadamsoftware, a.sidorin, Szelethus, donat.nagy, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D56917

llvm-svn: 351743