Check T for a trivial constructor when lowering default_construct_range.

Fixes #389
Simplify fallback_allocator example.

Fixes #358
Eliminate unused variable.
Merge pull request #462 from jaredhoberock/issue-358

Issue 358
Merge pull request #463 from jaredhoberock/issue-389

Check T for a trivial constructor when lowering default_construct_range.