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.

llvm-svn: 338074