[DebugInfo] Refactoring DIType::setFlags to DIType::cloneWithFlags, NFC

and using the latter in DIBuilder::createArtificialType and
DIBuilder::createObjectPointerType methods as well as introducing
mirroring DISubprogram::cloneWithFlags and
DIBuilder::createArtificialSubprogram methods.

The primary goal here is to add createArtificialSubprogram to support
a pass downstream while keeping the method consistent with the
existing ones and making sure we don't encourage changing already
created DI-nodes.

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D47615

llvm-svn: 333806