Rename variable to remove shadow warning on FiberManagerInternal.h

Summary: This variable shadows the field `options_`, so rename it to get rid of the warning `constructor parameter 'options_' shadows the field 'options_' of 'FiberManager'` (which is turned into an error because of `-Werror`). This happens on clang 8.

Reviewed By: yfeldblum

Differential Revision: D19331755

fbshipit-source-id: 3615cf9b6c7516775f1dc198a08bc784a52c0fb4

folly/fibers/FiberManagerInternal.h

@@ -143,8 +143,8 @@ class FiberManager : public ::folly::Executor {
    * with different Option, on the same EventBase.
    */
   struct FrozenOptions {
-    explicit FrozenOptions(Options options_)
-        : options(std::move(options_)), token(create(options)) {}
+    explicit FrozenOptions(Options opts)
+        : options(std::move(opts)), token(create(options)) {}
 
     const Options options;
     const ssize_t token;