Don't clang-format or clang-tidy subprojects

It's undesirable to try to format or tidy up subprojects for which we
aren't the authority on.  Making this change seems to allow one to use
"ninja clang-format" within a repo, and not have it reformat all the
subprojects.  More testing needed, but it's unlikely to hurt things.

Tested:
Ran "ninja clang-format" and observed _only_ the files in the repo get
formatted, not the subproject files.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ifda7dd02b114e404f33c51919a14ca1a2bbd2352
diff --git a/subprojects/.clang-tidy b/subprojects/.clang-tidy
new file mode 100644
index 0000000..612bd0e
--- /dev/null
+++ b/subprojects/.clang-tidy
@@ -0,0 +1 @@
+Checks: '-*'