use ImplicitSynchronized as appropriate

Summary:
To incrementally codemod away `folly::Synchronized`'s `operator->`,
and prevent new uses of `operator->`, migrate code that uses
`operator->` to `ImplicitSynchronized`.

See D48277297 and D32302068 for further context.

Reviewed By: Gownta

Differential Revision: D48278592

fbshipit-source-id: 09f9ae0f0411cede4f61279fe561620aae230f12

hphp/runtime/vm/jit/inlining-decider.cpp

@@ -319,7 +319,7 @@ Vcost computeTranslationCostSlow(SrcKey at,
   return irlower::computeIRUnitCost(*unit);
 }
 
-folly::Synchronized<InlineCostCache> s_inlCostCache;
+folly::ImplicitSynchronized<InlineCostCache> s_inlCostCache;
 
 int computeTranslationCost(SrcKey at,
                            const RegionDesc& region,