Auto-enable pretty-printing of libcxx in gdb. (#679)

.gdbinit

@@ -0,0 +1,6 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+source third_party/llvm-project/libcxx/utils/gdb/libcxx/printers.py
+python register_libcxx_printer_loader()