Added PersistentMap, an ADT that implements a map data structure that

Added PersistentMap, an ADT that implements a map data structure that
is persistent.  Adds/removals to a PersistentMap do not result in
a map being modified, but a new map being created.  This will be useful
for path-sensitive analyses.

The current implementation mainly makes copies to implement this
functionality.  If the map turns out to be extensively used, this
implementation will be replaced with a more efficient one that uses
data sharing (see comments in PersistentMap.h for more information).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@42290 91177308-0d34-0410-b5e6-96231b3b80d8