[cxxmodules] Enumerate all headers in GenVector.
[cxxmodules] Enumerate all headers in GenVector.

This patch reduces the duplicate content in the GenVector.pcm thus reducing
the amount of decl merging.

karies/root,root-mirror/root,olifre/root,karies/root,karies/root,olifre/root,olifre/root,karies/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,karies/root,olifre/root,karies/root,olifre/root,olifre/root,root-mirror/root,olifre/root,karies/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root