Remove TypeNodes.def from the modulemap.

We currently just look for files named in the modulemap in its
associated source directory.  This means that we can't name
generated files, like TypeNodes.def now is, which means we can't
explicitly mark it as textual.  But fortunately that's okay
because (as I understand it) the most important purpose of naming
the header in the modulemap is to ensure that it's not treated as
public, and the search for public headers also only considers
files in the associated source directory.  This isn't an elegant
solution, since among other things it means that a build which
wrote the generated files directly into the source directory would
result in something that wouldn't build as a module, but that's
a problem for all our other generated files as well.

llvm-svn: 373416