All directory targets now install the Makefile only if it is missing.

All directory targets now install the Makefile only if it is missing.
Directory targets no longer check for existance of the directory in the
object tree; if the Makefile doesn't exist, we will re-create the directory.
This seems to be a pretty good assumption and saves us from checking
directory existance each time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10211 91177308-0d34-0410-b5e6-96231b3b80d8