Link libc++demangle with libc++ for the device.

libc++demangle provides __cxa_demangle, which used to be a part of
libc++.so but is being moved out to save on resources since very few
libraries need it.

Test: make checkbuild
Bug: http://b/138245375
Change-Id: Ie97225d496d7d40f8749522bf36702a2d9dcdfe7
diff --git a/cc/testing.go b/cc/testing.go
index f0ad33b..e69b774 100644
--- a/cc/testing.go
+++ b/cc/testing.go
@@ -167,6 +167,16 @@
 			},
 		}
 		cc_library {
+			name: "libc++demangle",
+			no_libcrt: true,
+			nocrt: true,
+			system_shared_libs: [],
+			stl: "none",
+			host_supported: false,
+			vendor_available: true,
+			recovery_available: true,
+		}
+		cc_library {
 			name: "libunwind_llvm",
 			no_libcrt: true,
 			nocrt: true,