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/V3Class.cpp

@@ -257,7 +257,7 @@ public:
             modp->addStmtsp(nodep);
         }
         // BFS to mark public typedefs.
-        std::set<AstNodeUOrStructDType*> pubStrDtypeps;
+        std::set<const AstNodeUOrStructDType*> pubStrDtypeps;
         while (!m_pubStrDtypeps.empty()) {
             AstNodeUOrStructDType* const dtypep = m_pubStrDtypeps.front();
             m_pubStrDtypeps.pop();
@@ -270,7 +270,8 @@ public:
             }
         }
         for (AstTypedef* typedefp : m_typedefps) {
-            AstNodeUOrStructDType* const sdtypep = VN_AS(typedefp->dtypep(), NodeUOrStructDType);
+            const AstNodeUOrStructDType* const sdtypep
+                = VN_AS(typedefp->dtypep(), NodeUOrStructDType);
             if (pubStrDtypeps.count(sdtypep)) typedefp->attrPublic(true);
         }
         // Clear package pointer of non-public packed struct / union type, which will never be