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/V3Descope.cpp

@@ -55,11 +55,11 @@ class DescopeVisitor final : public VNVisitor {
 
     // METHODS
 
-    static bool modIsSingleton(AstNodeModule* modp) {
+    static bool modIsSingleton(const AstNodeModule* modp) {
         // True iff there's exactly one instance of this module in the design (including top).
         if (modp->isTop()) return true;
         int instances = 0;
-        for (AstNode* stmtp = modp->stmtsp(); stmtp; stmtp = stmtp->nextp()) {
+        for (const AstNode* stmtp = modp->stmtsp(); stmtp; stmtp = stmtp->nextp()) {
             if (VN_IS(stmtp, Scope)) {
                 if (++instances > 1) return false;
             }