Re-land "[CMake] Don't enable BUILD_WITH_INSTALL_RPATH when using custom build rpath"

[tambre]

Summary

Reverted check for empty CMAKE_BUILD_RPATH fixed.

When BUILD_WITH_INSTALL_RPATH is enabled it prevents using a custom rpath only
for the build tree as the install rpath will be used. This makes it impossible to run a
runtimes build when compiling with Clang and wanting the installed rpath to be
empty (i.e. -DCMAKE_BUILD_RPATH="<some path>" -DCMAKE_SKIP_INSTALL_RPATH=ON).

Disable BUILD_WITH_INSTALL_RPATH when CMAKE_BUILD_RPATH is non-empty to
allow for such build scenarios.

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(${name})
-  elseif (LLVM_LOCAL_RPATH)
-    set_target_properties(${name} PROPERTIES
-                          BUILD_WITH_INSTALL_RPATH On
-                          INSTALL_RPATH "${LLVM_LOCAL_RPATH}")
+  else()
+    # Enable BUILD_WITH_INSTALL_RPATH unless CMAKE_BUILD_RPATH is set.
+    if("${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()
 
   if(DEFINED windows_resource_file)
     set_windows_version_resource_properties(${name} ${windows_resource_file})
   endif()
 
   # $<TARGET_OBJECTS> doesn't require compile flags.
   if(NOT LLVM_ENABLE_OBJLIB)
if(LLVM_LINKER_IS_GNULD)
       # $ORIGIN is not interpreted at link time by ld.bfd
       set_property(TARGET ${name} APPEND_STRING PROPERTY
                    LINK_FLAGS " -Wl,-rpath-link,${LLVM_LIBRARY_OUTPUT_INTDIR} ")
     endif()
   else()
     return()
   endif()
 
+  # Enable BUILD_WITH_INSTALL_RPATH unless CMAKE_BUILD_RPATH is set.
+  if("${CMAKE_BUILD_RPATH}" STREQUAL "")
+    set_property(TARGET ${name} PROPERTY BUILD_WITH_INSTALL_RPATH ON)
+  endif()
+
   set_target_properties(${name} PROPERTIES
-                        BUILD_WITH_INSTALL_RPATH On
                         INSTALL_RPATH "${_install_rpath}"
                         ${_install_name_dir})
 endfunction()
 
 function(setup_dependency_debugging name)
   if(NOT LLVM_DEPENDENCY_DEBUGGING)
     return()
   endif()