Add a test that makes sure that in ./include and ./lib "#pragma clang optimize off" does not exist.

This is a useful debugging tool, but we really do not want it to sneak into the
code base by mistake.

Author: gottesmm

validation-test/banned_apis/banned_apis.test

@@ -0,0 +1,7 @@
+// This test runs a grep that validates that specific patterns do not occur in
+// the code base.
+
+// The [c] is necessary to prevent substitutions by lit. Same difference since it
+// is just a regex.
+//
+// RUN: cd %swift_src_root && not git grep 'pragma [c]lang optimize off' include lib stdlib