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/V3DiagSarif.cpp

@@ -33,14 +33,14 @@ class V3DiagSarifImp final {
 
     // METHODS
     void calculate() {
-        for (auto& msgr : m_messages)
+        for (const VErrorMessage& msgr : m_messages)
             if (msgr.code().isNamed()) m_codes.emplace(msgr.code());
         int i = 0;
-        for (const auto& code : m_codes) m_codeIndex[code] = i++;
+        for (const V3ErrorCode& code : m_codes) m_codeIndex[code] = i++;
     }
 
     void putRules(V3OutJsonFile& of) const {
-        for (const auto& code : m_codes) {
+        for (const V3ErrorCode& code : m_codes) {
             of.begin().put("id", code.ascii()).put("helpUri", code.url()).end();
         }
     }
@@ -97,7 +97,7 @@ class V3DiagSarifImp final {
         of.begin("message");
         putText(of, first_clean, first_fmt);
         of.end();
-        if (auto fl = msg.fileline()) {
+        if (const FileLine* const fl = msg.fileline()) {
             of.begin("locations", '[').begin();
             putLocation(of, fl);
             of.end();
@@ -175,7 +175,7 @@ public:
             .end();
 
         of.begin("results", '[');
-        for (auto& msgr : m_messages) putResult(of, msgr);
+        for (const VErrorMessage& msgr : m_messages) putResult(of, msgr);
         of.end();
 
         of.end();  // runs ]