[Attributor][modulemap] Revert r368064 but fix the build

Commit r368064 was necessary after r367953 (D65712) broke the module
build. That happened, apparently, because the template class IRAttribute
defined in the header had a virtual method defined in the corresponding
source file (IRAttribute::manifest). To unbreak the situation this patch
introduces a helper function IRAttributeManifest::manifestAttrs which
is used to implement IRAttribute::manifest in the header. The deifnition
of the helper function is still in the source file.

Patch by jdoerfert (Johannes Doerfert)

Differential Revision: https://reviews.llvm.org/D65821

llvm-svn: 368076