Simplify lambda parsing.

Simplify lambda parsing.

Since we removed the special parsing for C++11 lambdas, it's just been an
open-coded copy of cp_parser_function_body.  So let's call it instead.  This
avoids the need to change this code in my revised 33799 patch.

	* parser.c (cp_parser_lambda_body): Use cp_parser_function_body.