[clang-repl] Put CompilerInstance from IncrementalAction to use for non-assert/assert builds (#155400)

See
https://github.com/llvm/llvm-project/pull/137458#discussion_r2300649286

Context: So the CompilerInstance CI being used with the Incremental
Action are tightly coupled in any case. Which means
the  CI put to use while constructing the IncrementalAction
```
  Act = TSCtx->withContextDo([&](llvm::LLVMContext *Ctx) {
    return std::make_unique<IncrementalAction>(*CI, *Ctx, ErrOut, *this,
                                               std::move(Consumer));
  });                      
```

Is also the CI through which the we call `ExecuteAction` on `Act`
```
CI->ExecuteAction(*Act);
```

So we need to use CI as a member variable in IncrementalAction for
assert builds for

https://github.com/llvm/llvm-project/blob/bddac5eda9b7591e05ccdc86a5e86c592085f318/clang/lib/Interpreter/IncrementalAction.cpp#L97-L108

The same can be put to use for `CreateASTConsumer` too as all of these
are referring to the same CI
```
std::unique_ptr<ASTConsumer>
IncrementalAction::CreateASTConsumer(CompilerInstance & /*CI*/, StringRef InFile) {
  std::unique_ptr<ASTConsumer> C =
      WrapperFrontendAction::CreateASTConsumer(this->CI, InFile);
      
```