Override libcxxabi's .clang-format in the demangle directory

This directory uses LLVM style.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@344316 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/demangle/.clang-format b/src/demangle/.clang-format
new file mode 100644
index 0000000..5bead5f
--- /dev/null
+++ b/src/demangle/.clang-format
@@ -0,0 +1,2 @@
+BasedOnStyle: LLVM
+