Const suggestion check: build the Clang invocation in one function #249

Follow-up of #217 and #239 (plan Task 3.2, partly done).

The experimental const suggestion check (--experimental-const-suggestion-check) builds a Clang invocation from a unit's compile command in two places of src/analysis/ConstSuggestionFacts.cpp: the compilation that collects a unit's facts (inspect) and the identifier scan of the files of the compilation database. Both copy the arguments into argv, create the diagnostics and call clang::createInvocation with the same options; only the diagnostic consumer differs.

Build that invocation in one function used by both, reusing the existing path normalization of compile commands.

Not a cost fix: on the self-analysis the single construction was measured under 1 s, below the noise. The +10 % cost criterion of #217 is not met, and this follow-up does not change that.

Validation: outputs identical before and after (the suite, the T tests, the corpora with the option).