Refactor BitcodeReader: move Metadata and ValueId handling in their own class/file

Summary:
I'm planning on changing the way we load metadata to enable laziness.
I'm getting lost in this gigantic files, and gigantic class that is the bitcode
reader. This is a first toward splitting it in a few coarse components that
are more easily understandable.

Reviewers: pcc, tejohnson

Subscribers: mgorny, llvm-commits, dexonsmith

Differential Revision: https://reviews.llvm.org/D27646

llvm-svn: 289461