Fix crash when linking metadata with ODR type uniquing

Summary:
With DebugTypeODRUniquing enabled, during IR linking debug metadata
in the destination module may be reached from the source module.
This means that ConstantAsMetadata nodes (e.g. on DITemplateValueParameter)
may contain a value the destination module. When trying to map such
metadata nodes, we will attempt to map a GV already in the dest module.
linkGlobalValueProto will end up with a source GV that is the same as
the dest GV as well as the new GV. Trying to access the TypeMap for the
source GV type, which is actually a dest GV type, hits an assertion
since it appears that we have mapped into the source module (because the
type is the value not a key into the map).

Detect that we don't need to access the TypeMap in this case, since
there is no need to create a bitcast from the new GV to the source GV
type as they GV are the same.

Fixes PR35722.

Reviewers: mehdi_amini, pcc

Subscribers: probinson, llvm-commits, eraman

Differential Revision: https://reviews.llvm.org/D41624

llvm-svn: 322103