Remove lock from ConstString::GetLength

Summary: ConstStrings are immutable, so there is no need to grab even a reader lock in order to read the length field.

Reviewers: #lldb, labath

Reviewed By: labath

Subscribers: zturner, labath, lldb-commits

Differential Revision: https://reviews.llvm.org/D32306
Patch by Scott Smith <scott.smith@purestorage.com>

llvm-svn: 301642