instead of forcing blocks on by default, make them default to off, but let

instead of forcing blocks on by default, make them default to off, but let
specific targets default them to on.  Default blocks to on on 10.6 and later.
Add a -fblocks option that allows the user to override the target's default.
Use -fblocks in the various testcases that use blocks.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@60563 91177308-0d34-0410-b5e6-96231b3b80d8