build: Enable clang-tidy

Change-Id: Idbe531d78281889dd8b38b04d2e322e298f4558d
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/.clang-tidy-ignore b/.clang-tidy-ignore
new file mode 100644
index 0000000..a6b7f49
--- /dev/null
+++ b/.clang-tidy-ignore
@@ -0,0 +1,2 @@
+include/stdplus/str/cexpr.hpp
+test/str/cexpr.cpp