Replace std::to_string with operator<<(llvm::raw_ostream&)

[goldsborough]

This PR fixes #180 by replacing our overloads of std::to_string, which is UB, with corresponding llvm::raw_ostream& operator<<(llvm::raw_ostream&, <thing>) overloads.

This was less trivial than thought, because we were using a mix of std::ostream, std::stringstream and llvm::raw_ostream in the codebase. Furthermore, raw_ostream cannot output to files, while std::ostream can, so the replacement wasn't a simple find+replace. I think the best way forward is:

1. To make a class convertible to string, overload the operator<<(raw_ostream&),
2. For printing to stderr or stdout, use llvm::outs() and llvm::errs() as the stream source,
3. To construct a (formatted) string, use llvm::raw_string_ostream or llvm::raw_svector_stream and then call its .str() method,
4. For file output (which raw_ostream can't do), do (3) and to output it to a std::ofstream.

I think this PR also fixes a bug where we were calling std::to_string on a string literal (in examples/ptb.cpp), which was actually creating a string with the textual representation of the literal's memory address (because of the void* overload we had for std::to_string), rather than constructing a string.

I also recommend using one of my favorite LLVM data structures ever, llvm::Twine, for simple string concatenation instead of std::string::operator+.

It's a lot of code, so please do look at it carefully. Also, please tell me what would be a good test to make sure the dotty output for the IR looks good (@artemrakhov)?

[nadavrot]

The changes look good to me. Please run M.dumpDAG() and G.dumpDAG() and M.dump() and G.dump() and make sure that things work.

[goldsborough]

Tested the graph and IR dump and found a bug which I fixed. Generated dot graphs are now identical again so the patch is safe (hopefully).

[nadavrot]

Thank you!