Fix DtoCreateNestedContextType() for -linkonce-templates

[kinke]

Fixes #3690.

[kinke]

I can confirm that this fixes the issue for the executable mentioned in #3763 (comment). So both -link-defaultlib-shared and -linkonce-templates seem to work fine for a complex project on Windows now ('seem' because I haven't extensively tested it yet).

[kinke]

I haven't fully analyzed what happened before this change (generating the -vv output took something like half an hour on a fast box (as the issue only showed up when compiling the front-end all-at-once), and grepping an absolute horror due to https://issues.dlang.org/show_bug.cgi?id=21387), but seeing that irFunc.nestedContextCreated is set right before recursively walking up the parents hierarchy seemed dangerous wrt. reentrancy for the original function.

Crafting an according testcase would probably take significant effort.