Override #define of keyword

Summary:
Add a hidden option to suppress ###define [keyword]##, which is
occasionally appropriate.

Test Plan:
Build flint, run it on FBString.h, see no error.
Ran flint tests locally, see no errors.

Reviewed By: andrei.alexandrescu@fb.com

Subscribers: sdwilsh, louisk, njormrod, folly-diffs@

FB internal diff: D1644182

Tasks: 5486739

Signature: t1:1644182:1414616294:4b6004d358cba865455366f5a943644b3542ae51

folly/FBString.h

@@ -86,6 +86,7 @@
 
 // FBString cannot use throw when replacing std::string, though it may still
 // use std::__throw_*
+// nolint
 #define throw FOLLY_FBSTRING_MAY_NOT_USE_THROW
 
 #ifdef _LIBSTDCXX_FBSTRING