Another patch for modernize-loop-convert.

Summary:
1. Avoid converting loops that iterate over the size of a container and don't use its elements, as this would result in an unused-result warning.
2. Never capture the elements by value on lambdas, thus avoiding doing unnecessary copies and errors with non-copyable types.
3. The 'const auto &' instead of 'auto &' substitution on const containers now works on arrays and pseudoarrays as well.
4. The error about multiple replacements in the same macro call is now documented in the tests (not solved though).
5. Due to [1], I had to add a dummy usage of the range element (like "(void) *It;" or similars) on the tests that had empty loops.
6. I removed the braces from the CHECK comments. I think that there is no need for them, and they confuse vim.

Reviewers: klimek

Subscribers: alexfh, cfe-commits

Differential Revision: http://reviews.llvm.org/D12734

llvm-svn: 247399