[libc++] Fix leading zeros in std::to_chars

Summary:
It is a bugfix proposal for https://bugs.llvm.org/show_bug.cgi?id=42166.

`std::to_chars` appends leading zeros if input 64-bit value has 9, 10 or 11 digits.
According to documentation `std::to_chars` must not append leading zeros:
https://en.cppreference.com/w/cpp/utility/to_chars

Changeset should not affect `std::to_chars` performance:
http://quick-bench.com/CEpRs14xxA9WLvkXFtaJ3TWOVAg

Unit test that `std::from_chars` supports compatibility for both `std::to_chars` outputs (previous and fixed one) already exists:
https://github.com/llvm-mirror/libcxx/blob/1f60111b597e5cb80a4513ec86f79b7e137f7793/test/std/utilities/charconv/charconv.from.chars/integral.pass.cpp#L63

Reviewers: lichray, mclow.lists, ldionne, EricWF

Reviewed By: lichray, mclow.lists

Subscribers: zoecarver, christof, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D63047

llvm-svn: 362967