Remove byte counting from SourceManager [NFC]

Summary:
Similar to what we did in D50681, we now stop manually byte counting here
in the SourceManager.

Reviewers: #lldb, JDevlieghere

Reviewed By: #lldb, JDevlieghere

Subscribers: JDevlieghere, abidh, lldb-commits

Differential Revision: https://reviews.llvm.org/D50809

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@342121 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed