Override for include-guard

Summary:
In certain circumstances, it is appropriate to omit and include
guard. Add an option to allow this to be overriden.

Test Plan: Build flint, run it on folly, see no include-guard errors.

Reviewed By: andrei.alexandrescu@fb.com

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

FB internal diff: D1644170

Tasks: 5486739

Signature: t1:1644170:1414616595:7ac52f474c1312a0c28e89255b1151d56c680acf

folly/detail/AtomicHashUtils.h

@@ -18,6 +18,7 @@
 //
 // Note: no include guard; different -inl.h files include this and
 // undef it more than once in a translation unit.
+// override-include-guard
 
 #if !(defined(__x86__) || defined(__i386__) || FOLLY_X64)
 #define FOLLY_SPIN_WAIT(condition)                \

folly/test/FBStringTestBenchmarks.cpp.h

@@ -17,6 +17,8 @@
 /**
  * This file is supposed to be included from within
  * FBStringTest. Do not use otherwise.
+ *
+ * override-include-guard
  */
 
 void BENCHFUN(initRNG)(int iters, int) {