Fixing a cast-qual warning. getBufferStart() and getBufferEnd() both return a const char *, so casting to non-const was triggering a warning (even though the assignment and usage was always const anyway).

No functional changes intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207774 91177308-0d34-0410-b5e6-96231b3b80d8