dwarf2out: Fix up add_scalar_info not to create invalid DWARF

dwarf2out: Fix up add_scalar_info not to create invalid DWARF

As discussed in https://sourceware.org/bugzilla/show_bug.cgi?id=26987 ,
for very large bounds (which don't fit into HOST_WIDE_INT) GCC emits invalid
DWARF.
In DWARF2, DW_AT_{lower,upper}_bound were constant reference class.
In DWARF3 they are block constant reference and the
Static and Dynamic Properties of Types
chapter says:
"For a block, the value is interpreted as a DWARF expression; evaluation of the expression
yields the value of the attribute."
In DWARF4/5 they are constant exprloc reference class.
Now, for add_AT_wide we use DW_FORM_data16 (valid in constant class)
when -gdwarf-5, but otherwise just use DW_FORM_block1, which is not constant
class, but block.
For DWARF3 this means emitting clearly invalid DWARF, because the
DW_FORM_block1 should contain a DWARF expression, not random bytes
containing the constant directly.
For DWARF2/DWARF4/5 it could be considered a GNU extension, but a very badly
designed one when it means something different in DWARF3.

The following patch uses add_AT_wide only if we know we'll be using
DW_FORM_data16, and otherwise wastes 2 extra bytes and emits in there
DW_OP_implicit_value <size> before the constant.

2020-12-03  Jakub Jelinek  <jakub@redhat.com>

	* dwarf2out.c (add_scalar_info): Only use add_AT_wide for 128-bit
	constants and only in dwarf-5 or later, where DW_FORM_data16 is
	available.  Otherwise use DW_FORM_block*/DW_FORM_exprloc with
	DW_OP_implicit_value to describe the constant.