[CMake] Fix llvm_setup_rpath function

Summary:
Set _install_rpath to CMAKE_INSTALL_RPATH if it is defined, so that eventually
INSTALL_RPATH is set to CMAKE_INSTALL_RPATH.
The "if(NOT DEFINED CMAKE_INSTALL_RPATH)" was missing a corresponding else
clause.
This also cleans up the fix made in r285908.

Patch by Azharuddin Mohammed

Reviewers: john.brawn, sgundapa, beanz

Subscribers: chapuni, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D26289

llvm-svn: 286184