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

[Hubert Tong via llvm-commits]

================
@@ -572,6 +572,10 @@ else()
   option(LLVM_ENABLE_THREADS "Use threads if available." ON)
 endif()
 
+set(LLVM_ENABLE_ICU "OFF" CACHE STRING "Use ICU for character conversion support if available. Can be ON, OFF, or FORCE_ON")
+
+set(LLVM_ENABLE_ICONV "OFF" CACHE STRING "Use iconv for character conversion support if available. Can be ON, OFF, or FORCE_ON")
----------------
hubert-reinterpretcast wrote:

```suggestion
set(LLVM_ENABLE_ICONV "ON" CACHE STRING "Use iconv for character conversion support if available. Can be ON, OFF, or FORCE_ON")
```
My opinion is that we do want the `iconv` path enabled by default. For me, the main rationale is to increase the chances that at least half of the code being added is tested "widely".

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