[clang] [clang-repl] Handle mllvm args for clang-repl before calling executeAction (PR #197133)

[Anutosh Bhat via cfe-commits]

anutosh491 wrote:

Hey @vgvassilev, 

I updated the approach to use the common frontend path introduced around `CompilerInstance::PrepareForExecution()` (inspired by the prs you made on this like https://github.com/llvm/llvm-project/pull/192476)

So instead of having Clang-Repl call `parseLLVMArgs()` explicitly, `ExecuteAction()` now reaches it through `PrepareForExecution()`. This lets Clang-Repl and other direct CompilerInstance clients process `FrontendOpts::LLVMArgs` consistently. 

Added a test to  verify that -mllvm arguments are processed through ExecuteAction() exactly once, including for direct CompilerInstance clients like Clang-Repl.

https://github.com/llvm/llvm-project/pull/197133