DebugInfo: omit debug info for friends

GCC ToT doesn't do this & it's worth about 3.2% on Clang's DWO file size
with Clang. Some or all of this may be due to things like r190715 which
could have source fixes/improvements, but it's not clear that's the case
and that doesn't help other source bases.

llvm-svn: 190716