[bazel] Add TestingDemangle target for llvm#137947 (llvm#142419)

Added in f7e172d

Author: rupprecht

utils/bazel/llvm-project-overlay/llvm/BUILD.bazel

@@ -5163,6 +5163,13 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "TestingDemangle",
+    testonly = True,
+    copts = llvm_copts,
+    textual_hdrs = glob(["include/llvm/Testing/Demangle/*.inc"]),
+)
+
 cc_library(
     name = "TestingSupport",
     testonly = True,