Fix enum value lookups in C++

Fix enum value lookups in C++

The recent enum change was causing enums to break if used as constant
values by the C++ generator.  The problem was that we were searching for
the fully-qualified constant name (enum_name.VALUE_NAME) in the enum
values.  This didn't affect Java because it just uses symbolic names in
the generated code.  Fix it by grabbing the base name before doing the
search.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@996610 13f79535-47bb-0310-9956-ffa450edef68