[llvm] Create a CharSetConverter class with both iconv and icu support (PR #74516)

[Hubert Tong via llvm-commits]

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@@ -552,6 +552,10 @@ else()
   option(LLVM_ENABLE_THREADS "Use threads if available." ON)
 endif()
 
+set(LLVM_ENABLE_ICU "ON" CACHE STRING "Use ICU for character conversion support if available. Can be ON, OFF, or FORCE_ON")
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hubert-reinterpretcast wrote:

> These should default to OFF.

Both? The `iconv` support used is already limited to "built-in" support, e.g., from `libc`.

https://github.com/llvm/llvm-project/pull/74516