Darwin: Handle NULL DECL_SIZE_TYPE in machopic_select_section (PR94237).

Darwin: Handle NULL DECL_SIZE_TYPE in machopic_select_section (PR94237).

A recent change in the LTO streaming arrangement means that it is
now possible for machopic_select_section () to be called with a NULL
value for DECL_SIZE_TYPE - corresponding to an incomplete or not-yet-
laid out type.

When section anchors are present, and we are generating assembler, we
normally need to know the object size when choosing the section, since
zero-sized objects must be placed in sections that forbid section
anchors.

In the current circumstance, the objective of the earlier streaming of
this data is to allow nm to determine BSS c.f. Data symbols (when used
with the LTO plugin).  Since Darwin does not yet make use of the plugin
this fix is a bit of future-proofing.  We now emit the 'generic' section
for the decl (absent knowing its size) - which will still be correct in
determining the BSS c.f. Data case.

gcc/ChangeLog:

2020-03-21  Iain Sandoe  <iain@sandoe.co.uk>

	PR lto/94237
	* config/darwin.c (darwin_mergeable_constant_section): Collect
	section anchor checks into the caller.
	(machopic_select_section): Collect section anchor checks into
	the determination of 'effective zero-size' objects.  When the
	size is unknown, assume it is non-zero, and thus return the
	'generic' section for the DECL.