[Tooling] Fix FixedCompilationDatabase with header compile flags (#73913)

Summary:
The logic to strip positional args feels very fragile, but it's terribly
useful
when you want to use a tool on a file and have the exact argv.

Today doesn't work with header-parsing actions because these are
"precompile"
rather than "compile", from tooling's perspective it's all the same.

Reviewers:
kadircet

Subscribers: