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

Added in f7e172da4caeab9d92f6e97501b1a2c561e616c1
diff --git a/utils/bazel/llvm-project-overlay/llvm/BUILD.bazel b/utils/bazel/llvm-project-overlay/llvm/BUILD.bazel
index edc3976..e9807b21 100644
--- a/utils/bazel/llvm-project-overlay/llvm/BUILD.bazel
+++ b/utils/bazel/llvm-project-overlay/llvm/BUILD.bazel
@@ -5164,6 +5164,13 @@
 )
 
 cc_library(
+    name = "TestingDemangle",
+    testonly = True,
+    copts = llvm_copts,
+    textual_hdrs = glob(["include/llvm/Testing/Demangle/*.inc"]),
+)
+
+cc_library(
     name = "TestingSupport",
     testonly = True,
     srcs = glob([