Add temporary workaround to allow in-tree libc++ builds on Windows

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302753 91177308-0d34-0410-b5e6-96231b3b80d8

projects/CMakeLists.txt

@@ -22,7 +22,9 @@ endforeach(entry)
 if(${LLVM_BUILD_RUNTIME})
   # MSVC isn't quite working with libc++ yet, disable it until issues are
   # fixed.
-  if(NOT MSVC)
+  # FIXME: LLVM_FORCE_BUILD_RUNTIME is currently used by libc++ to force
+  # enable the in-tree build when targeting clang-cl.
+  if(NOT MSVC OR LLVM_FORCE_BUILD_RUNTIME)
     # Add the projects in reverse order of their dependencies so that the
     # dependent projects can see the target names of their dependencies.
     add_llvm_external_project(libunwind)