[CMake] Fix incorrect rpath for tests if LLVM_LOCAL_RPATH isn't set

[tambre]

Summary

d9ce31ae7d (D94322) removed the check because I thought it was dead
due to checking the existance of a variable (which always existed).

This causes LLDB tests to fail as they set NO_INSTALL_RPATH because
they're never meant to be installed, but we still would end up using
the install rpath.

Add the check back and make it explicitly check for an empty value
to make the purpose clearer and avoid implicit test for a false/true
value.

llvm/cmake/modules/AddLLVM.cmake

macro(add_llvm_executable name)
   else()
     add_executable(${name} ${ALL_FILES})
   endif()
 
   setup_dependency_debugging(${name} ${LLVM_COMMON_DEPENDS})
 
   if(NOT ARG_NO_INSTALL_RPATH)
     llvm_setup_rpath(${name})
-  else()
+  elseif(NOT "${LLVM_LOCAL_RPATH}" STREQUAL "")
     # Enable BUILD_WITH_INSTALL_RPATH unless CMAKE_BUILD_RPATH is set.
     if("${CMAKE_BUILD_RPATH}" STREQUAL "")
       set_property(TARGET ${name} PROPERTY BUILD_WITH_INSTALL_RPATH ON)
     endif()
 
     set_property(TARGET ${name} PROPERTY INSTALL_RPATH "${LLVM_LOCAL_RPATH}")
   endif()