Attaching comments to decls: since it was decided that Decl::isImplicit should

not be set for implicit instantiations, remove the FIXME.  This should be the
last bit for PR13634.  The actual fix happened in r162238.

Motivation: it might be misleading to mark implicit instantiations as
Decl::isImplicit = true.  Because then, in order to be consistent, we should
mark all instantiated members as implicit.  But the user did actually type the
declaration for the member, but the compiler played with it a little bit.

llvm-svn: 162488