[CMake] Create a custom target for generating extension module.

Previously we would create the extension module as a post-build
step of creating liblldb.  As part of this process, we created
symlinks and did other stuff.

This had several longstanding issues related to target
dependencies, such as not re-creating the symlink if liblldb.dll
failed to link, or if the build was Ctrl+C'ed from.

Then, the script that creates the symlinks began to grow to
include other things, such as argdumper, and it would try to
create symlinks before it had finished building the targets it
needed to symlink to.

This patches addresses all of these problems by creating an
explicit target for the script to run, and making it have a
dependency on all the targets it needs to create symlinks from/to.

llvm-svn: 229569

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