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

[Hubert Tong via llvm-commits]

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@@ -281,6 +281,12 @@
 /* Have host's ___chkstk_ms */
 #cmakedefine HAVE____CHKSTK_MS ${HAVE____CHKSTK_MS}
 
+/* Define if icu library is available */
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hubert-reinterpretcast wrote:

Nit: style
```suggestion
/* Define if ICU library is available */
```

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