[CMake] Don't enable BUILD_WITH_INSTALL_RPATH when using custom build rpath

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.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D93177

Author: tambry

llvm/cmake/modules/AddLLVM.cmake

@@ -865,10 +865,13 @@ macro(add_llvm_executable name)
 
   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 "${LLVM_LOCAL_RPATH}")
+  else()
+    # Enable BUILD_WITH_INSTALL_RPATH unless CMAKE_BUILD_RPATH is set.
+    if(NOT "${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)
@@ -2113,8 +2116,12 @@ function(llvm_setup_rpath name)
     return()
   endif()
 
+  # Enable BUILD_WITH_INSTALL_RPATH unless CMAKE_BUILD_RPATH is set.
+  if(NOT "${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()