Pretty simple.  Both flag-types.h and tree-core.h check to see if the 
other header has already defined this enumerated type, and defined it if 
not.

Its an unnecessary conditional compilation and dual maintenance of this 
small enum.  This patch puts it in coretypes.h and makes it always 
available.

Bootstraps on x86_64-pc-linux-gnu with no new regressions.  Ok for trunk?

Andrew