Fix it so that we only grab the typedef from the module DWARF file if the type...

Fix it so that we only grab the typedef from the module DWARF file if the type that is typedef'ed is a declaration. This fixes the following bugs:

<rdar://problem/26870890> [PR28156] TestWithModuleDebugging.py: failing on macOS
https://llvm.org/bugs/show_bug.cgi?id=27412
https://llvm.org/bugs/show_bug.cgi?id=28156

llvm-svn: 274809