llvm: conflict for CMake breaking change when treating ASM files as C (#20105)

var/spack/repos/builtin/packages/llvm/package.py

@@ -200,6 +200,11 @@ class Llvm(CMakePackage, CudaPackage):
             "system debug server",
     )
 
+    # LLVM bug https://bugs.llvm.org/show_bug.cgi?id=48234
+    # CMake bug: https://gitlab.kitware.com/cmake/cmake/-/issues/21469
+    # Fixed in upstream versions of both
+    conflicts('^cmake@3.19.0', when='@6.0.0:11.0.0')
+
     # Github issue #4986
     patch("llvm_gcc7.patch", when="@4.0.0:4.0.1+lldb %gcc@7.0:")
     # Backport from llvm master + additional fix