[Parser] complain for invalid enum raw value

Instead of fail silently, issue a diagosis when a valid expression can not
be parsed for enum raw value.

This resolves [SR-510](https://bugs.swift.org/browse/SR-510).

Author: dduan

include/swift/AST/DiagnosticsParse.def

@@ -584,6 +584,8 @@ ERROR(throw_in_function_type,type_parsing,none,
 // Enum Types
 ERROR(expected_expr_enum_case_raw_value,type_parsing,PointsToFirstBadToken,
       "expected expression after '=' in 'case'", ())
+ERROR(not_a_proper_raw_value_expression,type_parsing,none,
+      "not a proper raw value expression", ())
 ERROR(nonliteral_enum_case_raw_value,type_parsing,PointsToFirstBadToken,
       "raw value for enum case must be a literal", ())
 

lib/Parse/ParseDecl.cpp

@@ -4404,6 +4404,7 @@ ParserStatus Parser::parseDeclEnumCase(ParseDeclOptions Flags,
         return Status;
       }
       if (RawValueExpr.isNull()) {
+        diagnose(EqualsLoc, diag::not_a_proper_raw_value_expression);
         Status.setIsParseError();
         return Status;
       }