[CMake] Copy the generated __config header into build directory

When the generated __config file is being used, it is currently only
copied during installation process. However, that means that the file
that gets copied into LLVM build directory is the vanilla __config file,
and any parts of the build that depend on the just built toolchain like
sanitizers will get that instead of the generated version. To avoid this
issue, we need to copy the generated header into the LLVM build
directory as well.

Differential Revision: https://reviews.llvm.org/D43797

llvm-svn: 327194