Port Bazel toolchain and code to macOS (x86). (#238)

This updates the Bazel toolchain logic to work on macOS. Much like on
Linux, I'm not testing this against a *released* LLVM, but against
a from-source build. You can build and install LLVM from top-of-tree
locally with CMake, or on macOS maybe use Homebrew like:
```
brew install llvm --HEAD
```

You then need to point Bazel to the installed `clang` executable if it
is not placed onto your PATH (Homebrew doesn't):
```
bazel test --repo_env=CC=$HOME/homebrew/opt/llvm/bin/clang //parser:all
```

This builds and passes tests for me at least.

The fuzzer feature may not work (yet) with this setup, but that can be
improved incrementally as we proceed.

source/source_buffer.cpp

@@ -54,7 +54,12 @@ auto SourceBuffer::CreateFromFile(llvm::StringRef filename)
   }
 
   errno = 0;
-  void* mapped_text = mmap(nullptr, size, PROT_READ, MAP_PRIVATE | MAP_POPULATE,
+  void* mapped_text = mmap(nullptr, size, PROT_READ,
+#ifdef __APPLE__
+                           MAP_PRIVATE,
+#else
+                           MAP_PRIVATE | MAP_POPULATE,
+#endif
                            file_descriptor, /*offset=*/0);
   if (mapped_text == MAP_FAILED) {
     return ErrnoToError(errno);