Optimize typename caching

Building the typename cache (calling populateCache() in QDeclarativeCompiler::compileTree()) takes a significant portion of compile type (for simple components, can be up to 1/3 of the cost). The "common case" is for many components to build the same cache, so we should be able to drastically reduce the cost here.

This is primarily focused on caching of C++ components, but we might also need to consider what, if anything, needs to be done in the case of lots of non-C++ components (like in Qt Components).