[clang] [compiler-rt] [llvm] Migrate llvm-profdata to Opt flag parsing (PR #177868)

[David Zbarsky via cfe-commits]

dzbarsky wrote:

@petrhosek Do you also want to support `--foo=bar` form in addition to `--foo bar` form across flags? That's the other bit that changed to align with the previous migration PRs. I don't care one way or another, I just want to clear the path to merging this PR

https://github.com/llvm/llvm-project/pull/177868