I modified the StringMap that was being used to unique our debugger C strings

to have the value for the map be a "const char *" instead of an unused uint32_t.
This allows us to store the uniqued mangled/demangled counterpart in this map
for mangled names. This also speeds up the mangled/demangled counterpart lookup
that used to be maintained in a STL map by having direct access to the data.
If we eventually need to associate other strings to strings to more data, we
can make the value of the StringMap have a more complex value.

Added the start of a history source and history event class. It isn't being
used by anything yet, but might be shortly.

llvm-svn: 132813