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

[Hubert Tong via llvm-commits]

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@@ -257,6 +257,37 @@ else()
   set(LLVM_ENABLE_TERMINFO 0)
 endif()
 
+#Check for icu.
----------------
hubert-reinterpretcast wrote:

Nit: formatting/style
```suggestion
# Check for ICU.
```

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