[clang-scan-deps] Support double-dashes in clang command lines

This fixes argument injection in clang command lines, by adding them before "--".

Previously, the arguments were injected at the end of the command line and could be added after "--", which would be wrongly interpreted as input file paths.

This fix is needed for a subsequent patch, see D92191.

Differential Revision: https://reviews.llvm.org/D95099