clangsa-sast: make the cross-translation-unit (ctu) option explicit

Problem to solve

In Allow users to enable cross translation unit (C... (gitlab-org/security-products/analyzers/clangsa!26 - merged) • Jason Leasure • 18.2, support for cross translation unit (CTU) analysis (i.e., the --ctu option) was added to clangsa. However, enabling CTU currently requires configuring the analyzer through passthrough options.

Proposal

• Provide a dedicated command-line option in clangsa to enable cross translation unit analysis.
• Provide a configurable environment variable that, when set, enables cross translation unit analysis.