Const suggestion check: a failing original compilation is compiled again for each suggestion #252

Follow-up of #217 and #239 (final review, M4; plan Task 3.2).

With --experimental-const-suggestion-check, the original compilation of a unit other than the suggestion's is cached per file and configuration (originalOf in src/analysis/ConstSuggestionCheck.cpp). A compilation that fails is not cached: the std::optional stays empty, so the next suggestion that needs the same unit compiles it again and fails again.

On the self-analysis, 4 such compilations fail. The verdicts are right; only the cost is repeated.

Fix: cache the failure as well (for example a std::optional that holds a failed state), so each failing unit is compiled once per run.

Validation: same verdicts on the suite, the T tests and the corpora; the number of compilations drops.