Restore missing runtimes-test-depends target that causes build failures when LLVM_INCLUDE_TESTS is ON

[JamesNagurne]

Summary

Monorepo commit 7cc8377f, revision D121838 removed the 'runtimes-test-depends' target in runtimes builds that
is assumed to exist when using a bootstrapped runtime build.

For a full analysis, see:
https://discourse.llvm.org/t/looking-for-guidance-on-broken-downstream-bootstrapped-runtimes-builds/62934

The solution here is presented mostly as a stop-gap to get discussion
started, since contacting through Phabricator or Discourse have resulted
in no response.

A true solution would be to determine if runtimes-test-depends still makes
sense and, if not, remove it from the bootstrap runtimes CMakeLists.

[phosek]

LGTM

[ldionne]

runtimes/CMakeLists.txt

230

I can't find any mention of LLVM_RUNTIMES_LIT_DEPENDS in the monorepo anymore, so this will always be an empty target. It may "fix" the build in that it won't complain about a missing target anymore, but it doesn't look correct to me. @phosek ?

[JamesNagurne]

runtimes/CMakeLists.txt

230

This target is guaranteed by umbrella_lit_testsuite_begin

https://github.com/llvm/llvm-project/blob/4110c2c657b329f91a5c4eef653a15c1a15b61ed/llvm/cmake/modules/AddLLVM.cmake#L1888

I'm not a huge fan of relying on a variable that can't be easily searched, but this is the best facsimile of the original variable that was used as runtimes-test-depends' dependencies

[ldionne]

runtimes/CMakeLists.txt

230

Oh, wow, okay then.

runtimes/CMakeLists.txt

 # other runtimes are added.
 foreach(entry ${runtimes})
   get_filename_component(projName ${entry} NAME)
 
   add_subdirectory(${entry} ${projName})
 endforeach()
 
 if(LLVM_INCLUDE_TESTS)
+  # LLVM_RUNTIMES_LIT_DEPENDS is populated when lit tests are added between
+  # umbrella_list_testsuite begin and end. The bootstrap runtimes builds
+  # currently assumes this target exists.
+  add_custom_target(runtimes-test-depends ${LLVM_RUNTIMES_LIT_DEPENDS})
   # Add a global check rule now that all subdirectories have been traversed
   # and we know the total set of lit testsuites.
   umbrella_lit_testsuite_end(check-runtimes)
 
   if (NOT HAVE_LLVM_LIT)
     # If built by manually invoking cmake on this directory, we don't have
     # llvm-lit. If invoked via llvm/runtimes, the toplevel llvm cmake
     # invocation already generated the llvm-lit script.