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
GitOrigin-RevId: b7c374500607486d56ae16195b57ea737fa2e5a5
1 file changed