[libc++] Fixes std::to_chars for bases != 10.

While working on D70631, Microsoft's unit tests discovered an issue.
Our `std::to_chars` implementation for bases != 10 uses the range
`[first,last)` as temporary buffer. This violates the contract for
to_chars:
[charconv.to.chars]/1 http://eel.is/c++draft/charconv#to.chars-1
`to_chars_result to_chars(char* first, char* last, see below value, int base = 10);`
"If the member ec of the return value is such that the value is equal to
the value of a value-initialized errc, the conversion was successful and
the member ptr is the one-past-the-end pointer of the characters
written."

Our implementation modifies the range `[member ptr, last)`, which causes
Microsoft's test to fail. Their test verifies the buffer
`[member ptr, last)` is unchanged. (The test is only done when the
conversion is successful.)

While looking at the code I noticed the performance for bases != 10 also
is suboptimal. This is tracked in D97705.

This patch fixes the issue and adds a benchmark. This benchmark will be
used as baseline for D97705.

Reviewed By: #libc, Quuxplusone, zoecarver

Differential Revision: https://reviews.llvm.org/D100722