ValueMapper: fix assertion when null-mapping a constant for linking metadata

Summary:
When RF_NullMapMissingGlobalValues is set, mapValue can return null
for GlobalValue. When mapping the operands of a constant that is
referenced from metadata, we need to handle this case and actually
return null instead of mapping this constant.

Reviewers: dexonsmith, rafael

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20713

llvm-svn: 271129