[lldb] Fix that 'ninja clean' breaks the build by deleting debugserver_vers.c

Summary:
We mark debugserver_vers.c as a generated file in CMake. This means that when we run `ninja clean` we end up deleting that file,
but any following `ninja` invocation will fail due to the file missing. The file can't be generated as `ninja` doesn't know it has to
rerun CMake to create the file. Turns out that marking the output of configure_file as generated is wrong as explained in this bug report:
https://gitlab.kitware.com/cmake/cmake/issues/18032

This patch just removes that property. The only side effect of this seems to be that this file maybe shows up in your IDE when
opening our CMake project, but that seems like a small sacrifice.

This patch can be quickly tested by running `ninja clean ; ninja lldbDebugserverCommon`. Before this patch the build will fail
due to debugserver_vers.c missing.

Reviewers: JDevlieghere, labath

Reviewed By: labath

Subscribers: mgorny, abidh, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68376

llvm-svn: 373723