Added typedef "value_type" to DenseMap (similar typedef appears in std::map).

Added method FindAndConstruct() to DenseMap, which does the same thing as
operator[], except that it refers value_type& (a reference to both the
key and mapped data pair).  This method is useful for clients that wish
to access the stored key value, as opposed to the key used to do the
actual lookup (these need not always be the same).

Redefined operator[] to use FindAndConstruct() (same logic).

llvm-svn: 43594