[C++] Small optimizations

[mike-lischke]

Found a better way to manage s0 in the DFA if set from outside. And a few other small changes.

[jm-mikkelsen]

The change to Cpp.stg, removing the "antlr4::" qualifier on RuleContext, breaks unit tests for me. Tests with that change reverted running now, but passed all the tests that failed earlier. Example:

In file included from /tmp/BaseCppTest-main-1497246422711/L.cpp:5:
/tmp/BaseCppTest-main-1497246422711/L.h:52:17: error: unknown type name 'RuleContext'; did you mean 'antlr4::RuleContext'?
bool TSempred(RuleContext *_localctx, size_t predicateIndex);
^~~~~~~~~~~
antlr4::RuleContext
/home/janm/p4/amt5-io/antlr4/runtime-testsuite/target/classes/Cpp/runtime/src/RuleContext.h:62:26: note: 'antlr4::RuleContext' declared here
class ANTLR4CPP_PUBLIC RuleContext : public tree::ParseTree {
^
1 error generated.

[mike-lischke]

A pity that the Travis test constantly time out. That way you never see if you break the build. Thanks @jm-mikkelsen, I'll open a new PR to revert that change.