This commit fixes a problem identified in #251. When generating the
PROTOBUF_C__FORCE_ENUM_TO_BE_INT_SIZE directive at the end of the enum
values generated for a oneof, we inadvertently only included the
protobuf class name in the call to the directive. This breaks if there
is more than one oneof per protobuf class, which causes duplicate enum
names to be generated.
This issue is fixed by also appending the oneof's name to the protobuf
class name when generating the PROTOBUF_C__FORCE_ENUM_TO_BE_INT_SIZE
directive, which should cause unique enum names to be generated.
This changes the enum name that is generated, but these names are
implementation details since they are sentinel values that only exist to
force the compiler to size the enum type correctly.
(The test case for #220 is updated since that test relies on the
oneof implementation details in the code generator.)
