Add back lldb-framework-headers target
Add back lldb-framework-headers target

In r338058 we removed the target `lldb-framework-headers`, which mean
lldb-framework no longer depended on `framework_headers`, so they never
actually got generated. This is a partial revert of r338058: I added
back the lldb-framework-headers target, but the framework-header-fix.sh
script still runs on the copied headers.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@338074 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 7ec21c24c7d9a10bf5836561d5fee47f2883ebd3)

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb