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

[Petr Hosek via llvm-commits]

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@@ -257,6 +257,22 @@ else()
   set(LLVM_ENABLE_TERMINFO 0)
 endif()
 
+#Check for icu.
+find_package(ICU COMPONENTS uc i18n)
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petrhosek wrote:

Can we put this behind an option like `LLVM_ENABLE_ICU` (and similarly for iconv) akin to what we do for other dependencies such as zstd? ICU is a large dependency and some users may want to omit it even if it's available on their system (and conversely some vendors might want to fail the build if it's missing).

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