Modules: Separate out a checkSignature helper, almost NFC

The main point is to move the delete-the-new-module logic into the same block
that creates it, so I can simplify the memory management in a follow-up, but I
think it's clearer to use use a checkSignature helper here anyway.

There is a minor functionality change: we now scan ahead to pull the signature
out of the control block *only* if this is a new ModuleFile.  For old ones,
ASTReader::ReadControlBlock will have already read the signature.

llvm-svn: 293393