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

[Hubert Tong via llvm-commits]

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@@ -294,6 +294,41 @@ if(LLVM_HAS_LOGF128)
   set(LLVM_HAS_LOGF128 "${HAS_LOGF128}")
 endif()
 
+if (LLVM_ENABLE_ICU STREQUAL FORCE_ON AND LLVM_ENABLE_ICONV STREQUAL FORCE_ON)
+  message(FATAL_ERROR "LLVM_ENABLE_ICU and LLVM_ENABLE_ICONV should not both be FORCE_ON")
+endif()
+
+# Check for ICU. Only allow an optional, dynamic link for ICU so we don't impact LLVM's licensing.
+if(LLVM_ENABLE_ICU AND NOT(LLVM_ENABLE_ICONV STREQUAL FORCE_ON))
+  set(LIBRARY_SUFFIXES ${CMAKE_FIND_LIBRARY_SUFFIXES})
+  set(CMAKE_FIND_LIBRARY_SUFFIXES "${CMAKE_SHARED_LIBRARY_SUFFIX}")
+  if (LLVM_ENABLE_ICU STREQUAL FORCE_ON)
+    find_package(ICU REQUIRED COMPONENTS uc i18n)
+    if (NOT ICU_FOUND)
+      message(FATAL_ERROR "Failed to configure ICU, but LLVM_ENABLE_ICU is FORCE_ON")
+    endif()
+  else()
+    find_package(ICU COMPONENTS uc i18n)
+  endif()
+  set(HAVE_ICU ${ICU_FOUND})
+  set(CMAKE_FIND_LIBRARY_SUFFIXES ${LIBRARY_SUFFIXES})
+endif()
+
+# Check for builtin iconv to avoid licensing issues.
----------------
hubert-reinterpretcast wrote:

Minor nit.
```suggestion
# Check only for builtin iconv to avoid licensing issues.
```

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