Materialize metadata in IRLinker before value mapping

Summary:
Unless we plan to do later postpass metadata linking (ThinLTO special mode),
always invoke metadata materialization at the start of IRLinker::run().
This avoids the need for clients who use lazy metadata loading to
explicitly invoke materializeMetadata before the IRMover, which in
turn invokes IRLinker::run and needs materialized metadata for mapping.

Came up in the context of an LLD issue (D17982).

Reviewers: rafael

Subscribers: silvas, llvm-commits

Differential Revision: http://reviews.llvm.org/D17992

llvm-svn: 263143