Fixed warning from MingGW and error from Clang Windows

[ChaseOxide]

Reasons for changes are in commit messages.

In general, it fixes:

• In MingGW

  warning: declaration 'class std::exception' does not declare anything
  class ANTLR4CPP_PUBLIC std::exception; // Needed for VS 2015.
  

• In Clang (I tested with clang-tidy on Windows specifically)

  error: forward declaration of class cannot have a nested name specifier [clang-diagnostic-error]
    class ANTLR4CPP_PUBLIC std::exception; // Needed for VS 2015.
  

Side note: This sort of reverts the commit 19f584d, where the #ifdef flag for the forward declaration was removed.
Another note: Seems like there's more than just std::exception that requires forward declaration.

[parrt]

We are getting an error here:

E: Unable to locate package g++-5
E: Couldn't find any package by regex 'g++-5'
apt-get.diagnostics
apt-get install failed

Is that a spurious error or a real problem? @mike-lischke

[mike-lischke]

It's a pretty small patch. If that doesn't go through without warning we can leave it out. I'm surprised anyway, since I did not get such a warning in Visual Studio.

[parrt]

It looks to be a typical spurious inability to download a file from CI.