make FrozenRange work with std::string
make FrozenRange work with std::string

Summary:
We're having issues with non-fb platforms that are using `std::string`
(instead of `folly::fbstring`), as the former using wrapped iterators
(instead of raw pointers) that aren't implicitly convertible to `const char*`.

NOTE: This fix doesn't break non-string cases as `folly::Range` defines
comparison only for `folly::StringPiece` anyway.

Reviewed By: ddrcoder

Differential Revision: D3969763

fbshipit-source-id: cb847f970996e6b330c61c73aa31ef03027de665

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