BitcodeReader: Delay metadata parsing until reading a function body

There's hardly any functionality change here.  Instead of calling
materializeMetadata on the first call to materialize(GlobalValue*), wait
until the first one that's actually going to do something.  Noticed by
inspection; I don't have a concrete case where this makes a difference.

Added an assertion in materializeMetadata to be sure this (or a future
change) doesn't delay materializeMetadata after function-level metadata.

llvm-svn: 267345