D34444: Teach codegen to work in incremental processing mode.

When isIncrementalProcessingEnabled is on we might want to produce multiple
llvm::Modules. This patch allows the clients to start a new llvm::Module,
allowing CodeGen to continue working after a HandleEndOfTranslationUnit call.

This should give the necessary facilities to write a unittest for D34059.

As discussed in the review this is meant to give us a way to proceed forward
in our efforts to upstream our interpreter-related patches. The design of this
will likely change soon.

llvm-svn: 311843