DebugInfo: handle the DI asm printing change to reword '[fwd]' as '[decl]' and add '[def]' for non-decl tag types

This is to make test cases looking for definitions more legible by
making the definition explicit rather than just the absence of '[fwd]'.
This allowed the debug-info-record tests to be rephrased - and in the
interests of reducing the number of individual test cases/invocations we
have, I merged them into one file, separated them with namespaces (&
then moved them to C++ because namespaces are great). If they need to
remain 'C' only tests, they can be moved back. (I didn't group them with
'debug-info-class.cpp' because these tests only apply to
-fno-limit-debug-info)

I removed the pieces of code that would cause these tests to pass under
-flimit-debug-info to ensure the tests remain relevant to their fixes
should we ever improve -flimit-debug-info to catch that kind of code.

This commit is version locked with the corresponding change to
DebugInfo.h in LLVM. Except some transient buildbot fallout.
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.