thrift: clean up enum value assignment

[djwatson]

Summary:
Clean up how enum values are handled if an integer value is not
explicitly specified in the thrift file.

For example, the following used to be a compile error, but
works now:

  enum MyEnum {
    SOMEVALUE
  }
  struct MyStruct {
    1: MyEnum e = SOMEVALUE
  }

This change also cleans up some of the error handling with out-of-range
values.  Previously thrift simply issued a warning for enum values that
didn't fit in an i32, but serialized them as i32 anyway.  Now
out-of-range enum values result in a compile failure.

Test Plan:
Included a new unit test to verify the assignment of enum values.  I
also verified that g++ makes the same enum value assignments when
compiling these enums as C++ code.

[Jens-G]

There is no ticket yet, right?

[djwatson]

not yet

[bufferoverflow]

I made this: https://issues.apache.org/jira/browse/THRIFT-2513

testing...

[asfbot]

Build triggered. Test FAILed.