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

[Hubert Tong via llvm-commits]

================
@@ -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 "Both LLVM_ENABLE_ICU and LLVM_ENABLE_ICONV should not be FORCE_ON")
----------------
hubert-reinterpretcast wrote:

Minor wording nit:
```suggestion
  message(FATAL_ERROR "LLVM_ENABLE_ICU and LLVM_ENABLE_ICONV should not both be FORCE_ON")
```

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