I've made a first pass at adapting patches D17567 and D18035 to work
with your clang-3.8 packages. It builds and passes its own build-time
tests, and successfully builds and links a medium-size project we have
at work on 16.04, but I can't vouch for it much more than that.

I can see that this particular bug is attached to llvm-toolchain-3.6 and
not 3.8, but there doesn't seem to be a corresponding bug on 3.8, so
hopefully this isn't a bad place to mention it. Haven't tried patching
clang-3.6 yet.

** Patch added: "clang-3.8-abi_tag-ubuntu-changes.diff"
   
https://bugs.launchpad.net/ubuntu/+source/llvm-toolchain-3.6/+bug/1488254/+attachment/4703637/+files/clang-3.8-abi_tag-ubuntu-changes.diff

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https://bugs.launchpad.net/bugs/1488254

Title:
  clang++ no longer ABI-compatible with g++

Status in LLVM:
  Confirmed
Status in llvm-toolchain-3.6 package in Ubuntu:
  Confirmed

Bug description:
  $ cat foo.cc
  #include <string>
  std::string hello = "Hello, world!\n";
  $ cat bar.cc
  #include <string>
  #include <iostream>
  extern std::string hello;
  int main() {
      std::cout << hello;
      return 0;
  }
  $ g++ -c foo.cc && g++ foo.o bar.cc && ./a.out
  Hello, world!
  $ clang++ -c foo.cc && clang++ foo.o bar.cc && ./a.out
  Hello, world!
  $ g++ -c foo.cc && clang++ foo.o bar.cc && ./a.out
  /tmp/bar-34fb23.o: In function `main':
  bar.cc:(.text+0x14): undefined reference to `hello'
  clang: error: linker command failed with exit code 1 (use -v to see 
invocation)
  $ clang++ -c foo.cc && g++ foo.o bar.cc && ./a.out
  /tmp/ccqU38Mh.o: In function `main':
  bar.cc:(.text+0x5): undefined reference to `hello[abi:cxx11]'
  collect2: error: ld returned 1 exit status

  In practice, this means that many programs using C++ libraries other
  than libstdc++ fail to compile with clang++. For example, mosh fails
  with undefined references to
  google::protobuf::internal::empty_string_,
  google::protobuf::MessageLite::InitializationErrorString() const, and
  google::protobuf::MessageLite::SerializeAsString() const.

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