[PATCH] yacc.c: fix stack growth restriction for custom location types under C++

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In the GCC COBOL front end the main parser emits C code compiled as 
C++.  Sufficiently complex COBOL input to the parser requires stack
growth, and there is no documented way to enable that.  Empirically the
problem arises at 200 stack elements.  Some programs require over
1500.  

The Bison manual warns: 

1.  not to rely on undocumented macros, of which YYLTYPE_IS_TRIVIAL is
one
2.  any defined YYLTYPE must be trivially copyable

The attached patch merely allows the user to define YYLTYPE and
enforces the requirement that it be trivially-copyable.  

Humbly submitted.

--jkl

--

bison: Eliminate LTYPE_IS_TRIVIAL and test instead.

Enable stack-relocation when YYLTYPE is user-defined, and the emitted
C code is compiled as C++.

The undocumented variable LTYPE_IS_TRIVIAL is removed.  The guarded
stack-growth logic is emitted regardless of how YYLTYPE is defined, on
the theory that if %define api.location.type is used, the user has
read the manual and knows it requires a trivially-copyable type.

To prevent incorrectly copying a non-trivially-copyable YYLYTYPE, emit
a static assert against the type's trait.  If YYLTYPE fails to meet
the requirement, the code will not compile.

ChangeLog:

	* data/skeletons/yacc.c (if): Remove LTYPE_IS_TRIVIAL from the test.
	(static_assert): Require std::is_trivially_copyable.

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