Add travis-ci support

[dylanmckay]

Add travis-ci support to clang, and enable the -Werror flag to the compiler.

The travis build is tricky because clang must be built in the LLVM repository.

The .travis.yml file will probably be similar to lld/.travis.yml.

@RandomInsano would you like to do this? I am not well versed with travis.

[RandomInsano]

Can do. It'll take me a bit since work is going to take most of my week. If Clang and others are Git sub modules, I can probably just do a weird checkout of LLVM, then a submodule init before the build to pull everything in.

[RandomInsano]

Was closing this an oops? The referenced commit isn't right.

Also, I have some preliminary work done here. It's a busy weekend, so we'll see if I can hammer it out.

[dylanmckay]

You're right.

[neuralsandwich]

Any progress on adding travis support?