[clang-tidy] Handle unresolved expressions in ExprMutationAnalyzer

Summary:
- If a function is unresolved, assume it mutates its arguments
- Follow unresolved member expressions for nested mutations

Reviewers: aaron.ballman, JonasToth, george.karpenkov

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

Differential Revision: https://reviews.llvm.org/D50619

llvm-svn: 341848