BitcodeReader: Check for unresolved function metadata

A follow-up commit will start using function metadata blocks more
heavily.  This commit adds some error checking to confirm that metadata
is fully resolved before (and after) materializing each function.

This is valid even when reading very old bitcode from before the
metadata/value split.  The global metadata block always came before the
function blocks.  However, in case somehow this causes a regression
(i.e., an old LLVM did produce such bitcode after all) I'm committing
separately.

llvm-svn: 265223