Remove manual byte counting from Highlighter code.

Summary:
This removes the manual byte counting mechanism from the syntax highlighting
code. This is no longer necessary as the Stream class now has built-in support for
automatically counting the bytes that were written to it so far.

The advantage of automatic byte counting via Stream is that it is less error-prone
than the manual version and we need to write less boilerplate code.

Reviewers: labath

Reviewed By: labath

Subscribers: labath, lldb-commits

Differential Revision: https://reviews.llvm.org/D50676

llvm-svn: 339695