[clang] [clang-repl] Lay the foundation of pretty printing for C. (PR #89811)

[via cfe-commits]

zmodem wrote:

We're seeing the test fail on our mac builds: https://issues.chromium.org/issues/346289767

```
JIT session error: Symbols not found: [ ___clang_Interpreter_SetValueNoAlloc ]
 error: Failed to materialize symbols: { (main, { ____orc_init_func.incr_module_10, _c_str, $.incr_module_10.__inits.0 }) }
 error: Failed to materialize symbols: { (main, { ____orc_init_func.incr_module_10 }) }
```

Our current theory is that it's because we configure with `-DCLANG_PLUGIN_SUPPORT=OFF` and clang-repl has this in its cmakelists.txt:

https://github.com/llvm/llvm-project/blob/f26bc5f0c421c7a01a1a611249dd7483b749bab5/clang/tools/clang-repl/CMakeLists.txt#L64-L67

Maybe it should call `export_executable_symbols_for_plugins` unconditionally? Or the new test should `REQUIRES: plugins`?

https://github.com/llvm/llvm-project/pull/89811