Modules: Simplify the ModuleFile constructor; likely NFC

Zero-initialize ModuleFile members directly in the class definition, and
move the (now uninteresting) constructor into the class definition.

There were a few members that weren't being initialized at all.  I
zero-initialized them for consistency, but it's likely that they were
somehow initialized before their first use; i.e., there's likely no
functionality change here.

llvm-svn: 293404