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/V3Active.cpp

@@ -166,7 +166,7 @@ public:
     // paths make an assignment. Detected latches are flagged in the variables AstVar
     void latchCheck(AstNode* nodep, bool latch_expected) {
         bool latch_detected = false;
-        for (const auto& vrp : m_outputs) {
+        for (const AstVarRef* const vrp : m_outputs) {
             LatchDetectGraphVertex* const vertp = castVertexp(vrp->varp()->user1p());
             vertp->user(true);  // Identify the output vertex we are checking paths _to_
             if (!latchCheckInternal(castVertexp(vertices().frontp()))) latch_detected = true;