Fix bug in DW_OP_GNU_variable_value evaluation

A modified version of the gnat compiler (TBH I don't know if the
modifications are relevant to this bug or not, but I figured I'd
mention it) can generate a DWARF location expression like:

 <1><1201>: Abbrev Number: 3 (DW_TAG_dwarf_procedure)
    <1202>   DW_AT_location    : 32 byte block: 12 31 29 28 4 0 30 2f 12 0 14 30 2d 28 4 0 14 2f 1 0 30 34 1e 23 3 9 fc 1a 16 13 16 13 	(DW_OP_dup; DW_OP_lit1; DW_OP_eq; DW_OP_bra: 4; DW_OP_lit0; DW_OP_skip: 18; DW_OP_over; DW_OP_lit0; DW_OP_lt; DW_OP_bra: 4; DW_OP_over; DW_OP_skip: 1; DW_OP_lit0; DW_OP_lit4; DW_OP_mul; DW_OP_plus_uconst: 3; DW_OP_const1s: -4; DW_OP_and; DW_OP_swap; DW_OP_drop; DW_OP_swap; DW_OP_drop)

 <2><1279>: Abbrev Number: 9 (DW_TAG_structure_type)
    <127a>   DW_AT_name        : (indirect string, offset: 0x1a5a): p__logical_channel_t
    <127e>   DW_AT_byte_size   : 18 byte block: fd 43 12 0 0 97 94 1 99 34 0 0 0 23 7 9 fc 1a 	(DW_OP_GNU_variable_value: <0x1243>; DW_OP_push_object_address; DW_OP_deref_size: 1; DW_OP_call4: <0x1201>; DW_OP_plus_uconst: 7; DW_OP_const1s: -4; DW_OP_and)

When evaluated, this gives:

    Incompatible types on DWARF stack

In Jakub's original message about DW_OP_GNU_variable_value:

    https://gcc.gnu.org/legacy-ml/gcc-patches/2017-02/msg01499.html

.. it says:

    The intended behavior is that the debug info consumer computes the
    value of that referenced variable at the current PC, and if it can
    compute it and pushes the value as a generic type integer into the
    DWARF stack

Instead, gdb is using the variable's type -- but this fails with some
operations, like DW_OP_and, which expect the types to match.

I believe what was intended was for the value to be cast to the DWARF
"untyped" type, which is what this patch implements.  This patch also
updates varval.exp to exhibit the bug.

gdb/ChangeLog
2020-07-28  Tom Tromey  <tromey@adacore.com>

	* dwarf2/expr.c (dwarf_expr_context::execute_stack_op)
	<DW_OP_GNU_variable_value>: Cast to address type.

gdb/testsuite/ChangeLog
2020-07-28  Tom Tromey  <tromey@adacore.com>

	* gdb.dwarf2/varval.exp (setup_exec): Add 'or' instruction to
	'varval' location.