Move classes from Core -> Utility.

This moves the following classes from Core -> Utility.

ConstString
Error
RegularExpression
Stream
StreamString

The goal here is to get lldbUtility into a state where it has
no dependendencies except on itself and LLVM, so it can be the
starting point at which to start untangling LLDB's dependencies.
These are all low level and very widely used classes, and
previously lldbUtility had dependencies up to lldbCore in order
to use these classes.  So moving then down to lldbUtility makes
sense from both the short term and long term perspective in
solving this problem.

Differential Revision: https://reviews.llvm.org/D29427

llvm-svn: 293941