Avoid memory leaks by not invoking longjmp in yyreportAmbiguity.

Avoid memory leaks by not invoking longjmp in yyreportAmbiguity.
* tests/glr-regression.at (Leaked semantic values when reporting
ambiguity): New test case.
* data/glr.c (yyreportAmbiguity): Invoke yyyerror directly and return
yyabort rather than invoking yyFail, which invokes longjmp.  Remove the
now unnecessary yystackp parameter.
(yyresolveValue): Return yyreportAmbiguity's result.  Now the necessary
destructors can be called.

* tests/glr-regression.at: Don't invoke bison with `-t' unnecessarily
in existing testcases.