d: Fix undefined reference to nested lambda in template (PR108055)

Sometimes, nested lambdas of templated functions get no code generation
due to them being marked as instantianted outside of all modules being
compiled in the current compilation unit.  This despite enclosing
template instances being marked as instantiated inside the current
compilation unit.  To fix, all enclosing templates are now checked in
`function_defined_in_root_p'.

Because of this change, `function_needs_inline_definition_p' has also
been fixed up to only check whether the regular function definition
itself is to be emitted in the current compilation unit.

	PR d/108055

gcc/d/ChangeLog:

	* decl.cc (function_defined_in_root_p): Check all enclosing template
	instances for definition in a root module.
	(function_needs_inline_definition_p): Replace call to
	function_defined_in_root_p with test for outer module `isRoot'.

gcc/testsuite/ChangeLog:

	* gdc.dg/torture/imports/pr108055conv.d: New.
	* gdc.dg/torture/imports/pr108055spec.d: New.
	* gdc.dg/torture/imports/pr108055write.d: New.
	* gdc.dg/torture/pr108055.d: New test.