clang-format-vs : Fix Unicode formatting

Use UTF-8 for communication with clang-format and convert the
replacements offset/length to characters position/count.

Internally VisualStudio.Text.Editor.IWpfTextView use sequence of Unicode
characters encoded using UTF-16 and use characters position/count for
manipulating text.

Resolved "Error while running clang-format: Specified argument was out
of the range of valid values. Parameter name: replaceSpan".

Patch by empty2fill!

Differential revision: https://reviews.llvm.org/D70633