Internals: Improve cppcheck flow and fix up issues (#6311)

Added cppcheck-suppressions.txt in the repo root. You can add new
patterns in there instead of having to parse the XML output.

Also configure to add the -D__GNUC__ preprocessor macro, which makes it
understand UASSERT (it understands the 'noreturn' function attribute).

Added some case by case specific suppressions and fixed up other code,
especially in V3Ast*h and V3Dfg*.h, including code generated by astgen
that had some no-ops that irks cppcheck.

One thing it does not seem to like is `const` class members with default
initializers in the class. It will assume that's always the value, even
if overridden in the constructor. We had few so removed them.

With that a lot of files in `src/` are now clean or only have a handful
of issues. Therefore, I have also deleted cppcheck_filtered, and made it
produce human readable output straight to the terminal.

Regarding cleaning up the reported nits, I kind of got bored after
V3[A-E] so pausing here. Apologies for the merge conflicts.

Tested with cppcheck 2.13.0

src/V3CfgLiveVariables.cpp

@@ -83,7 +83,7 @@ class CfgLiveVariables final : VNVisitorConst {
     // Check and return if variable is incompatible
     bool incompatible(Variable* varp) {
         if (!isSupportedPackedDType(varp->dtypep())) return true;
-        AstVar* astVarp = nullptr;
+        const AstVar* astVarp = nullptr;
         // TODO: remove the useless reinterpret_casts when C++17 'if constexpr' actually works
         if VL_CONSTEXPR_CXX17 (T_Scoped) {
             astVarp = reinterpret_cast<AstVarScope*>(varp)->varp();