Fix compile error with libstdc++.

By adding a ctor to create fuzzer_allocator<T> from fuzzer_allocator<U>.
This mimics construcotrs of std::allocator<T>.

Without the constructors, some versions of libstdc++ can't compile
`vector<bool, fuzzer_allocator<bool>>`.

llvm-svn: 334077

compiler-rt/lib/fuzzer/FuzzerDefs.h

@@ -155,6 +155,11 @@ extern ExternalFunctions *EF;
 template<typename T>
   class fuzzer_allocator: public std::allocator<T> {
     public:
+      fuzzer_allocator() = default;
+
+      template<class U>
+      fuzzer_allocator(const fuzzer_allocator<U>&) {}
+
       template<class Other>
       struct rebind { typedef fuzzer_allocator<Other> other;  };
   };