[clang-tidy] performance-unnecessary-copy-initialization: Fix false negative.

`isConstRefReturningMethodCall` should be considering
`CXXOperatorCallExpr` in addition to `CXXMemberCallExpr`. Clang considers
these to be distinct (`CXXOperatorCallExpr` derives from `CallExpr`, not
`CXXMemberCallExpr`), but we don't care in the context of this
check.

This is important because of
`std::vector<Expensive>::operator[](size_t) const`.

Differential Revision: https://reviews.llvm.org/D114249