Reduce size of FileLine

Multiple tricks to reduce the size of class FileLine from 72 to 40
bytes:

- Reduce file name index from 32 to 16 bits. This still allows 64K
  unique input files, which is hopefully enough.
- Intern message/warning enable bitset and use a 16-bit index, again
  allowing 64K unique sets which is hopefully enough.
- Put the m_waive flag into the sign bit of one of the line numbers.
- Use explicit reference counting to avoid overhead of shared_ptr.

Added assertions to ensure interned data fits within it's index space.

This saves ~5-10% peak memory consumption at no measurable run-time cost
on various designs.

src/V3Param.cpp

@@ -101,7 +101,7 @@ public:
             for (V3HierarchicalBlockOption::ParamStrMap::const_iterator pIt = params.begin();
                  pIt != params.end(); ++pIt) {
                 std::unique_ptr<AstConst> constp{AstConst::parseParamLiteral(
-                    new FileLine(FileLine::EmptySecret()), pIt->second)};
+                    new FileLine{FileLine::builtInFilename()}, pIt->second)};
                 UASSERT(constp, pIt->second << " is not a valid parameter literal");
                 const bool inserted = consts.emplace(pIt->first, std::move(constp)).second;
                 UASSERT(inserted, pIt->first << " is already added");