[clang-tidy] Fix tests for performance-for-range-copy

Test failed as D52120 made ExprMutationAnalyzer smarter, fixed by:
- Add move-ctor for `Mutable` to make it actually movable.
- Properly implement `remove_reference`.

The failed test case is:
void negativeVarIsMoved() {
  for (auto M : View<Iterator<Mutable>>()) {
    auto Moved = std::move(M);
  }
}
Before D52120, `std::move(M)` itself is considered as a mutation to `M`,
while after D52120 it's only considered as a cast to rvalue, the
move-assignment is what causes the actual mutation. The test case didn't
mock things properly so the intended move-assignement was actually a
copy-assignment.

llvm-svn: 342417