When an "inline" declaration was followed by a definition not marked

When an "inline" declaration was followed by a definition not marked
"inline", we weren't giving the definition weak linkage because the
"inline" bit wasn't propagated. This was a longstanding FIXME that,
somehow, hadn't triggered a bug in the wild. Fix this problem by
tracking whether any declaration was marked "inline", and clean up the
semantics of GNU's "extern inline" semantics calculation based on this
change.

Fixes <rdar://problem/8740363>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@121373 91177308-0d34-0410-b5e6-96231b3b80d8