Where does the __1 symbol come from when using LLVM's libc++?

I see a fair amount of questions like Apple Mach-O Linker (Id) Error and Undefined symbols in cryptopp at IOS 64-bit project. The problem is usually described as:

Undefined symbols for architecture i386:
  "std::__1::basic_ostream<char, std::__1::char_traits<char> >::flush()", referenced from:
      cv::gpu::error(char const*, char const*, int, char const*) in opencv2(gpumat.o)

The problem often reduces to mixing/matching -stdlib=libc++ (LLVM C++ runtime) and -stdlib=libstdc++ (GNU C++ runtime). The LLVM C++ runtime (libc++) has an __1 decoration symbol, but the GNU C++ runtime libstdc++ lacks the __1 symbol in its name. It causes linker problems for symbols that appears to have the same name (like std::string).

Where does the __1 symbol come from when using LLVM's libc++?

Why was the problem not solved with a gnu namespace and an llvm namespace?

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Here's a related question: libc++ - stop std renaming to std::__1?. But it kind of misses the point in that a rename does not occur.