Remove manual byte counting from internal Stream methods.

Summary:
This patch removes the manual byte counting in all internal Stream methods.
This is now done by the automatic byte counting provided by calling `GetWrittenBytes()`
before and after writing the data (which is automatically done for us by the `ByteDelta`
utility class).

Reviewers: #lldb, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: JDevlieghere, labath, lldb-commits

Differential Revision: https://reviews.llvm.org/D50681

llvm-svn: 342044