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

[Sergei Barannikov via llvm-commits]

================
@@ -236,6 +236,12 @@
 /* Have host's ___chkstk_ms */
 #cmakedefine HAVE____CHKSTK_MS ${HAVE____CHKSTK_MS}
 
+/* Define if ICU library is available */
+#cmakedefine HAVE_ICU ${HAVE_ICU}
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s-barannikov wrote:

(nit)
```suggestion
#cmakedefine01 HAVE_ICU
```


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