[clang-tidy] UnnecessaryCopyInitialization - Extend to trigger on non-const...

[clang-tidy] UnnecessaryCopyInitialization - Extend to trigger on non-const "this" object argument if it is not modified.

Summary:

Also trigger the check in the following case:

void foo() {
  ExpensiveToCopy Obj;
  const auto UnnecessaryCopy = Obj.constReference();
  Obj.onlyUsedAsConst();
}

i.e. when the object the method is called on is not const but is never
modified.

Reviewers: alexfh, fowles

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20010

llvm-svn: 271239