DebugInfo: elide type index entries for synthetic types

Compiler emitted synthetic types may not have an associated DIFile
(translation unit).  In such a case, when generating CodeView debug type
information, we would attempt to compute an absolute filepath which
would result in a segfault due to a NULL DIFile*.  If there is no source
file associated with the type, elide the type index entry for the type
and record the type information.  This actually results in higher
fidelity debug information than clang/C2 as of this writing.

Resolves PR32668!

llvm-svn: 302085