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

[Hubert Tong via llvm-commits]

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@@ -295,6 +296,14 @@ add_llvm_component_library(LLVMSupport
   Demangle
   )
 
+# Link icu library if it is an external library.
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hubert-reinterpretcast wrote:

Nit: style
```suggestion
# Link ICU library if it is an external library.
```

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