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/V3CoverageJoin.cpp

@@ -48,7 +48,7 @@ class CoverageJoinVisitor final : public VNVisitor {
         // Note uses user4
         V3DupFinder dupFinder;  // Duplicate code detection
         // Hash all of the original signals we toggle cover
-        for (AstCoverToggle* nodep : m_toggleps) dupFinder.insert(nodep->origp());
+        for (const AstCoverToggle* const nodep : m_toggleps) dupFinder.insert(nodep->origp());
         if (dumpLevel() || debug() >= 9)
             dupFinder.dumpFile(v3Global.debugFilename("coveragejoin") + ".hash", false);
         // Find if there are any duplicates