DebugInfo: When asm printing include a '[def]' tag for tag decls that are definitions (& rename the 'fwd' tag to 'decl' for clarity)

This change is version locked with a change in Clang, so expect some
transient buildbot fallout.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184525 91177308-0d34-0410-b5e6-96231b3b80d8