[CPP] Cache keep growing and will not be reset while parsing multiple files, just behaves like memory leak

Reference: https://github.com/antlr/antlr4/issues/499
At first I thought it was memory leak, however I finally find out that it is a case that the "cache" will not be release and soon grow to hundreds megabytes(with keeping the reference to a lot of ATNConfig and DFAState) after parsing some files.

I come up with a way to bypass that, like this, in grammar files for both lexer and parser:
```
@lexer::members {
void cleanCache() {
_interpreter->clearDFA();
_sharedContextCache.clear();
}
}
```
and call this method every time before I parse a file.

Maybe something like this method should be provided by runtime library, since the cache members are both static private and I cannot find any document about this problem.

Start by tracing the C++ runtime entry points mentioned in the report: _interpreter->clearDFA() and _sharedContextCache.clear(). Compare their behavior when parsing multiple files and determine how a supported cache-reset API should expose the needed cleanup. Done means the cache can be reset through the runtime without grammar-local workarounds and the reported growth is covered by an appropriate test.

Written by the indexing model from the issue text.