gccgo-6 installation breaks the Clang compiler

[max ulidtko]

Steps to reproduce:

1) Install Clang

  $ sudo apt-get update && sudo apt-get install -y clang

2) Compile and run a C++ helloworld

  $ cat > hello.cxx << EOF
  #include <iostream>
  int main() { std::cout << "hello world\n"; }
  EOF

  $ clang++ hello.cxx -o hello && ./hello
  hello world

3) Install gccgo-6

  $ sudo apt-get install -y gccgo-6

4) Compile and run the same helloworld again

  $ clang++ hello.cxx -o hello && ./hello
  hello.cxx:1:10: fatal error: 'iostream' file not found
  #include <iostream>
         ^
1 error generated.

Actual result: the error message above. Expected result: clang++ continues to work.

Tested in a clean fresh Docker container.

# lsb_release -rdc
Description: Ubuntu 16.04.1 LTS
Release: 16.04
Codename: xenial