Use inline namespaces with GCC instead of __attribute__((__strong__)).

GCC 7.0.1 started warning that __attribute__((__strong__)) is depricated.
This patch switches to using inline namespace with GCC instead. I believe
this wasn't done originally in order to support older GCC versions w/o
support for inline namespaces, or because earlier versions of GCC warned
users that the STL was using an inline namespace (even though it shouldn't affect users).

However I believe all of the above problems are gone for GCC 4.9 and greater.
Therefore switching to using inline namespaces instead of using __strong__
is the most correct behavior.

llvm-svn: 295428