[llvm] Create a EncodingConverter class with both iconv and icu support. (PR #138893)

[Hubert Tong via llvm-commits]

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+//===-- EncodingConverter.h - Encoding conversion class -----------*- C++ -*-=//
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hubert-reinterpretcast wrote:

Is it worthwhile perhaps to say "TextEncoding" (for the filename and other names) instead of plain "Encoding"?

https://github.com/llvm/llvm-project/pull/138893