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

[Hubert Tong via llvm-commits]

================
@@ -592,6 +592,10 @@ else()
   option(LLVM_ENABLE_THREADS "Use threads if available." ON)
 endif()
 
+set(LLVM_ENABLE_ICU "OFF" CACHE STRING "Use ICU for character conversion support if available. Can be ON, OFF, or FORCE_ON")
+
+set(LLVM_ENABLE_ICONV "OFF" CACHE STRING "Use iconv for character conversion support if available. Can be ON, OFF, or FORCE_ON")
----------------
hubert-reinterpretcast wrote:

```suggestion
set(LLVM_ENABLE_ICU "OFF" CACHE STRING "Use ICU for text encoding conversion support if available. Can be ON, OFF, or FORCE_ON")

set(LLVM_ENABLE_ICONV "OFF" CACHE STRING "Use iconv for text encoding conversion support if available. Can be ON, OFF, or FORCE_ON")
```

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