Make any header files that are automatically generated be preconditions of
the compilation. This ensures that if a *.h.in file is changed then its
corresponding *.h file gets updated on the next rebuild. Note that this can
lead to confusing (but correct) results if the *.h.in file changed
unsubstantially so that autoheader doesn't update the *.h file. In that case,
manually touch the *.h file in question to restore order. Moral of the story,
if you're going to "touch" a *.in file then modify it substantially.

llvm-svn: 23006