Fix infinite loop when processing unknown double-dash argument

Paragraph 4.1 (framaCIRGen specific options) of the frama-clang documentation states that the parameters `--info`, `--warn` and `--debug` might be used to enable extra messages at runtime. They don't appear to be implemented right now. Instead, execution gets stuck in an infinite loop.
Status Job ID Name Coverage
  Git Update
failed #424999
nix
git-update

 
  Tests
skipped #425001
allowed to fail manual
tests-on-llvm-10
skipped #425002
nix
tests-on-llvm-11
skipped #425000
nix
tests-on-llvm-9
 
Name Stage Failure
failed
git-update Git Update There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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