IRMover: Avoid accidentally mapping types from the destination module (PR30799)

IRMover: Avoid accidentally mapping types from the destination module (PR30799)

During Module linking, it's possible for SrcM->getIdentifiedStructTypes();
to return types that are actually defined in the destination module
(DstM). Depending on how the bitcode file was read,
getIdentifiedStructTypes() might do a walk over all values, including
metadata nodes, looking for types. In my case, a debug info metadata
node was shared between the two modules, and it referred to a type
defined in the destination module (see test case).

Differential Revision: https://reviews.llvm.org/D26212

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287353 91177308-0d34-0410-b5e6-96231b3b80d8