c++: Defer emitting inline variables [PR113708]

Inline variables are vague-linkage, and may or may not need to be
emitted in any TU that they are part of, similarly to e.g. template
instantiations.

Currently 'import_export_decl' assumes that inline variables have
already been emitted when it comes to end-of-TU processing, and so
crashes when importing non-trivially-initialised variables from a
module, as they have not yet been finalised.

This patch fixes this by ensuring that inline variables are always
deferred till end-of-TU processing, unifying the behaviour for module
and non-module code.

	PR c++/113708

gcc/cp/ChangeLog:

	* decl.cc (make_rtl_for_nonlocal_decl): Defer inline variables.
	* decl2.cc (import_export_decl): Support inline variables.

gcc/testsuite/ChangeLog:

	* g++.dg/debug/dwarf2/inline-var-1.C: Reference 'a' to ensure it
	is emitted.
	* g++.dg/debug/dwarf2/inline-var-3.C: Likewise.
	* g++.dg/modules/init-7_a.H: New test.
	* g++.dg/modules/init-7_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>