Make framework-header-fix process copied headers

Summary:
Previously the framework-header-fix script would change the sources
before they were copied, leading to unnecessary rebuilds on repeat
`ninja lldb` invocations. This runs the script on the headers after
they're copied into the produced LLDB.framework, meaning it doesn't
affect any files being built.

Patch by Keith Smiley <keithbsmiley@gmail.com>!

Differential Revision: https://reviews.llvm.org/D49779

llvm-svn: 338058