[clang-tidy] Handle sugared reference types in ExprMutationAnalyzer

Summary:
This handles cases like this:
```
typedef int& IntRef;
void mutate(IntRef);
void f() {
  int x;
  mutate(x);
}
```
where the param type is a sugared type (`TypedefType`) instead of a
reference type directly.

Note that another category of similar but different cases are already
handled properly before:
```
typedef int Int;
void mutate(Int&);
void f() {
  int x;
  mutate(x);
}
```

Reviewers: aaron.ballman, alexfh, george.karpenkov

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

Differential Revision: https://reviews.llvm.org/D50953

llvm-svn: 341986