[clang] [clang-repl] Handle frontend options for clang-repl before calling executeAction (PR #132670)

[Anutosh Bhat via cfe-commits]

anutosh491 wrote:

I see clang handle 4 types of FrontendOpts before calling ExecuteAction

1) help
2) version
3) Plugins through [LoadRequestedPlugins](https://github.com/llvm/llvm-project/blob/4e4e4a190fb7c74453994935c843b09cc682f4bb/clang/lib/FrontendTool/ExecuteCompilerInvocation.cpp#L225)
4) llvmargs

We might be interesting in 1, 2 and 4 for clang-repl
Also clang-repl uses IncrementalAction unlike FrontendAction so simply calling ExecuteCompilerInvocation won't do the job. So not sure if we can maybe refactor out 1/2/4 and use it from clang itself.

https://github.com/llvm/llvm-project/pull/132670