Cleaned up the StringLexer a little bit.  It turns
out we only want to roll back text that was in the
buffer to begin with, so it's not necessary to
provide a pushback stack.

I'm going to use this slightly cleaner API to perform
lookahead for the Objective-C runtime type parser.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@221640 91177308-0d34-0410-b5e6-96231b3b80d8