Change UniqueCStringMap to use ConstString as the key

Summary:
UniqueCStringMap "sorts" the entries for fast lookup, but really it only cares about uniqueness.  ConstString can be compared by pointer alone, rather than with strcmp, resulting in much faster comparisons.  Change the interface to take ConstString instead, and propagate use of the type to the callers where appropriate.

Reviewers: #lldb, clayborg

Reviewed By: clayborg

Subscribers: labath, jasonmolenda, lldb-commits

Differential Revision: https://reviews.llvm.org/D32316
Patch by Scott Smith <scott.smith@purestorage.com>.

llvm-svn: 301908