make folly::Range literal type

make folly::Range literal type

Summary:
7.1.5 [dcl.constexpr] / 9 (N3337) requires type to be literal to use it
in `constexpr` object declaration (seems to be not enforced by GCC 4.8?)

Currently `folly::Range<>` fails one of the requirements:

* is an aggregate type, or has at least one constexpr constructor or
constructor template that is not a copy or move constructor,

Test Plan: fbconfig folly/test:range_test && fbmake runtests_opt -j32

Reviewed By: lucian@fb.com

Subscribers: chaoyc, search-fbcode-diffs@, unicorn-diffs@

FB internal diff: D1441646

Tasks: 4720575