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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190716 91177308-0d34-0410-b5e6-96231b3b80d8