[clang-tidy] ExprMutationAnalyzer: construct from references. Fixes PR38888

Summary:
I have hit this the rough way, while trying to use this in D51870.

There is no particular point in storing the pointers, and moreover
the pointers are assumed to be non-null, and that assumption is not
enforced. If they are null, it won't be able to do anything good
with them anyway.

Initially i thought about simply adding asserts() that they are
not null, but taking/storing references looks like even cleaner solution?

Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=38888 | PR38888 ]]

Reviewers: JonasToth, shuaiwang, alexfh, george.karpenkov

Reviewed By: shuaiwang

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

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D51884

llvm-svn: 341854