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[REG2.068.1] Issue 15021 - linker error with speculative instantiation and -inline

Refer the detailed explanation in `test/runnable/ice15030.d`.

Sadly today, `TemplateInstance::enclosing == NULL` is not equivalent with "the template instance is not nested".
Because of the inconsistency, setting enclosing will break processes in other places.

[REG2.068.1] Issue 15030 - ICE with recursive delegate, -unittest, and std.range

Conflicts:
	src/opover.c
	test/runnable/sdtor.d

Conflicts:
	src/access.d
	src/aggregate.d
	src/aliasthis.d
	src/apply.d
	src/argtypes.d
	src/arrayop.d
	src/arraytypes.d
	src/attrib.d
	src/builtin.d
	src/canthrow.d
	src/clone.d
	src/cond.d
	src/constfold.d
	src/cppmangle.d
	src/ctfeexpr.d
	src/dcast.d
	src/dclass.d
	src/declaration.d
	src/delegatize.d
	src/denum.d
	src/dimport.d
	src/dinterpret.d
	src/dmacro.d
	src/dmangle.d
	src/dmodule.d
	src/doc.d
	src/dscope.d
	src/dstruct.d
	src/dsymbol.d
	src/dtemplate.d
	src/dversion.d
	src/errors.d
	src/escape.d
	src/expression.d
	src/func.d
	src/globals.d
	src/hdrgen.d
	src/identifier.d
	src/init.d
	src/inline.d
	src/json.d
	src/lexer.d
	src/lib.d
	src/link.d
	src/mars.d
	src/mtype.d
	src/nogc.d
	src/nspace.d
	src/objc.d
	src/opover.d
	src/optimize.d
	src/parse.d
	src/root/file.d
	src/root/man.d
	src/root/outbuffer.d
	src/sapply.d
	src/sideeffect.d
	src/statement.d
	src/staticassert.d
	src/target.d
	src/tokens.d
	src/traits.d
	src/visitor.d

Merge branch 'merge_stable_convert' into merge_stable