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

[Anutosh Bhat via cfe-commits]

anutosh491 wrote:

Had to revert the latest commit for now !!

I totally missed that some direct `ExecuteAction()` clients, particularly `clang-scan-deps`, intentionally carry`-mllvm ` options without registering the corresponding backend flags. Moving the parsing into the common path caused those tools to reject otherwise valid options, so the narrower Clang-Repl-specific handling is safer !

See CI for errors. It shows 
```
Unknown command line argument '-stackmap-version=2'
Unknown command line argument '-x86-asm-syntax=intel'
Unknown command line argument '-enable-memcpyopt-without-libcalls'
```

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